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Invalid template file in module ”

I just installed a new theme, and as soon as I go to the front-end I get this exception:

1 exception(s):
Exception #0 (MagentoFrameworkExceptionValidatorException): Invalid template file: 'Magento_Theme::newletter-popup.phtml' in module: '' block's name: 'home.script'

Exception #0 (MagentoFrameworkExceptionValidatorException): Invalid template file: 'Magento_Theme::newletter-popup.phtml' in module: '' block's name: 'home.script'
#1 MagentoFrameworkViewElementTemplate->_toHtml() called at [lib/internal/Magento/Framework/View/Element/AbstractBlock.php:1110]
#2 MagentoFrameworkViewElementAbstractBlock->MagentoFrameworkViewElement{closure}() called at [lib/internal/Magento/Framework/View/Element/AbstractBlock.php:1114]
#3 MagentoFrameworkViewElementAbstractBlock->_loadCache() called at [lib/internal/Magento/Framework/View/Element/AbstractBlock.php:674]
#4 MagentoFrameworkViewElementAbstractBlock->toHtml() called at [lib/internal/Magento/Framework/View/Layout.php:566]
#5 MagentoFrameworkViewLayout->_renderBlock('home.script') called at [lib/internal/Magento/Framework/View/Layout.php:542]
#6 MagentoFrameworkViewLayout->renderNonCachedElement('home.script') called at [generated/code/Magento/Framework/View/Layout/Interceptor.php:206]
#7 MagentoFrameworkViewLayoutInterceptor->renderNonCachedElement('home.script') called at [lib/internal/Magento/Framework/View/Layout.php:497]
#8 MagentoFrameworkViewLayout->renderElement('home.script', false) called at [generated/code/Magento/Framework/View/Layout/Interceptor.php:193]
#9 MagentoFrameworkViewLayoutInterceptor->renderElement('home.script', false) called at [lib/internal/Magento/Framework/View/Layout.php:594]
#10 MagentoFrameworkViewLayout->_renderContainer('page.wrapper', false) called at [lib/internal/Magento/Framework/View/Layout.php:544]
#11 MagentoFrameworkViewLayout->renderNonCachedElement('page.wrapper') called at [generated/code/Magento/Framework/View/Layout/Interceptor.php:206]
#12 MagentoFrameworkViewLayoutInterceptor->renderNonCachedElement('page.wrapper') called at [lib/internal/Magento/Framework/View/Layout.php:497]
#13 MagentoFrameworkViewLayout->renderElement('page.wrapper', false) called at [generated/code/Magento/Framework/View/Layout/Interceptor.php:193]
#14 MagentoFrameworkViewLayoutInterceptor->renderElement('page.wrapper', false) called at [lib/internal/Magento/Framework/View/Layout.php:594]
#15 MagentoFrameworkViewLayout->_renderContainer('root', false) called at [lib/internal/Magento/Framework/View/Layout.php:544]
#16 MagentoFrameworkViewLayout->renderNonCachedElement('root') called at [generated/code/Magento/Framework/View/Layout/Interceptor.php:206]
#17 MagentoFrameworkViewLayoutInterceptor->renderNonCachedElement('root') called at [lib/internal/Magento/Framework/View/Layout.php:497]
#18 MagentoFrameworkViewLayout->renderElement('root', true) called at [generated/code/Magento/Framework/View/Layout/Interceptor.php:193]
#19 MagentoFrameworkViewLayoutInterceptor->renderElement('root') called at [lib/internal/Magento/Framework/View/Layout.php:963]
#20 MagentoFrameworkViewLayout->getOutput() called at [lib/internal/Magento/Framework/Interception/Interceptor.php:58]
#21 MagentoFrameworkViewLayoutInterceptor->___callParent('getOutput', array()) called at [lib/internal/Magento/Framework/Interception/Interceptor.php:138]
#22 MagentoFrameworkViewLayoutInterceptor->MagentoFrameworkInterception{closure}() called at [lib/internal/Magento/Framework/Interception/Interceptor.php:153]
#23 MagentoFrameworkViewLayoutInterceptor->___callPlugins('getOutput', array(), array(array('layout-model-cac...'))) called at [generated/code/Magento/Framework/View/Layout/Interceptor.php:494]
#24 MagentoFrameworkViewLayoutInterceptor->getOutput() called at [lib/internal/Magento/Framework/View/Result/Page.php:257]
#25 MagentoFrameworkViewResultPage->render(&MagentoFrameworkAppResponseHttpInterceptor#0000000060b00286000000003b09d81b#) called at [lib/internal/Magento/Framework/View/Result/Layout.php:170]
#26 MagentoFrameworkViewResultLayout->renderResult(&MagentoFrameworkAppResponseHttpInterceptor#0000000060b00286000000003b09d81b#) called at [lib/internal/Magento/Framework/Interception/Interceptor.php:58]
#27 MGSMpanelBlockFrameworkPageInterceptor->___callParent('renderResult', array(&MagentoFrameworkAppResponseHttpInterceptor#0000000060b00286000000003b09d81b#)) called at [lib/internal/Magento/Framework/Interception/Interceptor.php:138]
#28 MGSMpanelBlockFrameworkPageInterceptor->MagentoFrameworkInterception{closure}(&MagentoFrameworkAppResponseHttpInterceptor#0000000060b00286000000003b09d81b#) called at [lib/internal/Magento/Framework/Interception/Interceptor.php:153]
#29 MGSMpanelBlockFrameworkPageInterceptor->___callPlugins('renderResult', array(&MagentoFrameworkAppResponseHttpInterceptor#0000000060b00286000000003b09d81b#), array(array('result-messages', 'result-builtin-c...', 'result-varnish-c...'))) called at [generated/code/MGS/Mpanel/Block/Framework/Page/Interceptor.php:156]
#30 MGSMpanelBlockFrameworkPageInterceptor->renderResult(&MagentoFrameworkAppResponseHttpInterceptor#0000000060b00286000000003b09d81b#) called at [lib/internal/Magento/Framework/App/Http.php:120]
#31 MagentoFrameworkAppHttp->launch() called at [generated/code/Magento/Framework/App/Http/Interceptor.php:24]
#32 MagentoFrameworkAppHttpInterceptor->launch() called at [lib/internal/Magento/Framework/App/Bootstrap.php:260]
#33 MagentoFrameworkAppBootstrap->run(&MagentoFrameworkAppHttpInterceptor#0000000060b0029d000000003b09d81b#) called at [index.php:39]

When I try to go to admin, instead it gives me this:

1 exception(s):
Exception #0 (MagentoFrameworkExceptionLocalizedException): Invalid Document 
Element 'add': Duplicate key-sequence ['Magento_Backend::mgs'] in unique identity-constraint 'uniqueAddItemId'.
Line: 25


Exception #0 (MagentoFrameworkExceptionLocalizedException): Invalid Document 
Element 'add': Duplicate key-sequence ['Magento_Backend::mgs'] in unique identity-constraint 'uniqueAddItemId'.
Line: 25

#1 MagentoFrameworkConfigReaderFilesystem->read('adminhtml') called at [app/code/Magento/Backend/Model/Menu/Config.php:144]
#2 MagentoBackendModelMenuConfig->_initMenu() called at [app/code/Magento/Backend/Model/Menu/Config.php:111]
#3 MagentoBackendModelMenuConfig->getMenu() called at [app/code/Magento/Backend/Model/Url.php:365]
#4 MagentoBackendModelUrl->_getMenu() called at [app/code/Magento/Backend/Model/Url.php:325]
#5 MagentoBackendModelUrl->getStartupPageUrl() called at [app/code/Magento/Backend/App/AbstractAction.php:186]
#6 MagentoBackendAppAbstractAction->_processUrlKeys() called at [generated/code/Magento/Backend/Controller/Adminhtml/Index/Index/Interceptor.php:50]
#7 MagentoBackendControllerAdminhtmlIndexIndexInterceptor->_processUrlKeys() called at [app/code/Magento/Backend/App/Request/BackendValidator.php:175]
#8 MagentoBackendAppRequestBackendValidator->validate(&MagentoFrameworkAppRequestHttp#000000002dd898420000000046a9b4a0#, &MagentoBackendControllerAdminhtmlIndexIndexInterceptor#000000002dd89b3c0000000046a9b4a0#) called at [lib/internal/Magento/Framework/App/Request/CompositeValidator.php:40]
#9 MagentoFrameworkAppRequestCompositeValidator->validate(&MagentoFrameworkAppRequestHttp#000000002dd898420000000046a9b4a0#, &MagentoBackendControllerAdminhtmlIndexIndexInterceptor#000000002dd89b3c0000000046a9b4a0#) called at [lib/internal/Magento/Framework/App/FrontController.php:160]
#10 MagentoFrameworkAppFrontController->processRequest(&MagentoFrameworkAppRequestHttp#000000002dd898420000000046a9b4a0#, &MagentoBackendControllerAdminhtmlIndexIndexInterceptor#000000002dd89b3c0000000046a9b4a0#) called at [lib/internal/Magento/Framework/App/FrontController.php:118]
#11 MagentoFrameworkAppFrontController->dispatch(&MagentoFrameworkAppRequestHttp#000000002dd898420000000046a9b4a0#) called at [lib/internal/Magento/Framework/Interception/Interceptor.php:58]
#12 MagentoFrameworkAppFrontControllerInterceptor->___callParent('dispatch', array(&MagentoFrameworkAppRequestHttp#000000002dd898420000000046a9b4a0#)) called at [lib/internal/Magento/Framework/Interception/Interceptor.php:138]
#13 MagentoFrameworkAppFrontControllerInterceptor->MagentoFrameworkInterception{closure}(&MagentoFrameworkAppRequestHttp#000000002dd898420000000046a9b4a0#) called at [lib/internal/Magento/Framework/Interception/Interceptor.php:153]
#14 MagentoFrameworkAppFrontControllerInterceptor->___callPlugins('dispatch', array(&MagentoFrameworkAppRequestHttp#000000002dd898420000000046a9b4a0#), array(array('default_store_se...', 'page_cache_from_...', 'storeCookieValid...', 'install', 'configHash'))) called at [generated/code/Magento/Framework/App/FrontController/Interceptor.php:26]
#15 MagentoFrameworkAppFrontControllerInterceptor->dispatch(&MagentoFrameworkAppRequestHttp#000000002dd898420000000046a9b4a0#) called at [lib/internal/Magento/Framework/App/Http.php:116]
#16 MagentoFrameworkAppHttp->launch() called at [generated/code/Magento/Framework/App/Http/Interceptor.php:24]
#17 MagentoFrameworkAppHttpInterceptor->launch() called at [lib/internal/Magento/Framework/App/Bootstrap.php:260]
#18 MagentoFrameworkAppBootstrap->run(&MagentoFrameworkAppHttpInterceptor#000000002dd898540000000046a9b4a0#) called at [index.php:39]

Furthermore, when I run setup compile,

Fatal error:  Declaration of MGSMpanelControllerCategoryView::execute() must be compatible with MagentoCatalogControllerCategoryView::execute(): ?MagentoFrameworkControllerResultInterface in /var/www/html/Magento2/app/code/MGS/Mpanel/Controller/Category/View.php on line 129