i have error in main page about the Theme

[soha_gamal]

i have this error in main page 

i don't know how i can solve it any one can help please ?

Fatal error: Uncaught Error: Call to a member function getId() on null in /home/haalsite/public_html/vendor/magento/framework/View/Model/Layout/Merge.php:751 Stack trace: #0 /home/haalsite/public_html/vendor/magento/framework/View/Model/Layout/Merge.php(684): Magento\Framework\View\Model\Layout\Merge->_getPhysicalTheme(Object(Magento\Theme\Model\Theme)) #1 /home/haalsite/public_html/vendor/magento/framework/View/Model/Layout/Merge.php(633): Magento\Framework\View\Model\Layout\Merge->_loadFileLayoutUpdatesXml() #2 /home/haalsite/public_html/var/generation/Magento/Framework/View/Model/Layout/Merge/Interceptor.php(232): Magento\Framework\View\Model\Layout\Merge->getFileLayoutUpdatesXml() #3 /home/haalsite/public_html/vendor/magento/framework/View/Model/Layout/Merge.php(527): Magento\Framework\View\Model\Layout\Merge\Interceptor->getFileLayoutUpdatesXml() #4 /home/haalsite/public_html/vendor/magento/framework/View/Model/Layout/Merge.php(506): Magento\Framework\View\Model\Layout\Merge->_fetchPackageLayoutUpdates('default') #5 in /home/haalsite/public_html/vendor/magento/framework/View/Model/Layout/Merge.php on line 751

[kalyanchakri]

It is difficult to help just with this stack trace. Do you know at least from when you are facing this issue or after which change it started happening?