[desktop_drop_web] Dragging file from zip file crashes

[jtmcdole]

Reproduce Steps

1. Drag a file from a zip file (windows explorer)
2. Observer error when releasing.

Expected behavior

No errors

Version (please complete the following information):

• Flutter Version: master
• OS: Windows + Chrome
• plugin: [e.g. desktop_drop: 0.7.0]

Stack trace:

errors.dart:274 Uncaught DartError: Unexpected null value.
    at Object.throw_ [as throw] (errors.dart:274:3)
    at Object.nullCheck (operations.dart:950:18)
    at desktop_drop_web.dart:92:46
    at List.generate (core_patch.dart:377:46)
    at desktop_drop_web.dart:90:24
    at Object._callDartFunctionFast1 (js_allow_interop_patch.dart:224:27)
    at ret (js_allow_interop_patch.dart:84:15)

[schindler-s]

We recently had the same problem when dragging a file from the mail client (colleague using Windows/Chrome). On my mac I couldn't drag files from outlook to the app so I tried it using our project management tool and could reproduce it there.

It seems this issue appears whenever a file isn't dragged from file explorer.

The error appears due to the ! when getting the entry.

Future.wait(List.generate(items.length, (index) {
        final item = items[index];
        final entry = item.webkitGetAsEntry()!;
        return _entryToWebDropItem(entry);
      })).then((webItems) {
        channel.invokeMethod(
          "performOperation_web",
          webItems.map((e) => e.toJson()).toList(),
        );
      }).catchError((e, s) {
        debugPrint('desktop_drop_web: $e $s');
      });

Flutter Version: master
OS: MacOs/Windows + Chrome

Edit: I added some prints to get the content of the files being recognized

Future.wait(List.generate(items.length, (index) {
        final item = items[index];
        print(item.type); // <- here
        item.getAsString(((JSString text) => print(text.toDart)).toJS); // <- here
        final entry = item.webkitGetAsEntry()!;
        return _entryToWebDropItem(entry);
      })).then((webItems) {
        channel.invokeMethod(
          "performOperation_web",
          webItems.map((e) => e.toJson()).toList(),
        );
      }).catchError((e, s) {
        debugPrint('desktop_drop_web: $e $s');
      });

Result when dragging an image file from PM-Tool:

text/html
<meta http-equiv="Content-Type" content="text/html;charset=UTF-8"><img src="someRandomURL" title="image.png" width="656" height="134" data-pm-slice="0 0 []">

"item.webkitGetAsEntry()" only returns a value if the DataTransferItem is a file which is not true in our cases