Fix rare exception in webview on Android

[borisyankov]

Rendering some messages (very rarely, but consistently) throw an error on Android.

[borisyankov]

It seems the error happens when we send some html messages to the phone (when transitioning from 'loading' to 'some messages loaded' state).
When the messages already exist, they are rendered properly.

[gnprice]

@borisyankov says this issue may be related to #2505 and #2558.

[gnprice]

Fixed in 787c4a5 (from #2854) a couple of weeks ago, along with #2505 and #2558.