[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)