Avoid null pointer exception after webview is disposed

[robsonmeemo]

Fixes #104

[peter-meemo]

IIUC, the webview being gone at this point means that we cannot pass a value back from dart into the javascript; this PR will stop the error message but not resolve that issue.

[robsonmeemo]

But I think it's the responsibility of the library user to keep the webview around for long enough to allow the javascript side to deal with it. Maybe make an extra javascript handler call to dart to signal you're done and the webview can be closed.