ios: Copy-paste into Zulip is broken when using Handoff

[chrisbobbe]

A user reported in beta feedback:

On iOS 16.7.10, the first time pasting into each of the hostname/user/password fields a dialog shows asking to allow the paste but then disappears before being clicked. Only on the second paste (per field) the dialog can be clicked.

They confirmed that they were pasting via the Handoff feature (which is used e.g. to copy-paste from a laptop to an iPhone). So this is a match for an upstream Flutter issue: flutter/flutter#121446