========== Update ==========
I make some special test for this today. Even if I check this type, it still cause error. Because the datatransfer.items have two items, the datatransfer.files have only one item, fox example: I drag one image to webpage, it check first item and skip it, then it's over because of datatransfer.files have only one item. so webpage can't get this file. that's so weird. you can check this 4.png attachment.
========== Origin ===========
There are two items "datatransferitem" when only drop one image file to webpage. if I don't check kind of file that will cause null error when invoke directly item.webkitGetAsEntry().isDirectory property.
It's only one item and work fine in chromium. In github there are many js components not check these files type, therefore, I think you should fix this problem as soon as possible, or else 5.7 drag and drop feature will face many compatibility issues, that's so terrible if occur.
There is other problem also prevent drag and drop file to webpage, I don't know if qtwebengine occur this error, but in chromium it's work fine.
following is my test website and image of error info: