Fix crash when prompt was called on Android Q.

[AlexV525]

Connection with issue(s)

Connected to #261 .

Testing and Review Notes

1. Visit a web page included window.prompt (e.g. https://www.w3schools.com/jsref/tryit.asp?filename=tryjsref_prompt).
2. Call window.prompt then the app crashed.

To Do

• test with more cases

[arnaudelub]

If you want, as the maintainer is out for quite a long time now, we are maintaining this fork:
https://github.com/arnaudelub/flutter_polywebview
package also available on pub.dev:
https://pub.dev/packages/flutter_polywebview

feel free to add you PR on this fork if you want.