This is a common issue in wordpress 5.2 and many users have experienced it. There are lot of solutions and tutorial how to solve it. Some suggest to activate debug mode, deactivate plugins, change themes and etc.
Sometimes you follow all the above steps and you still have the smae issue. That’s where you need to check this solution. You also don’t need to follow all above steps and can jump to this one to check if it can solve your issue. For me, It always solved my proble.
You need to download your theme zip file from wordpress website and upload it manually. Then activate the theme again. This simple approach solves the problem in many cases.