Fix crashes when removing spaces in mention texts

[SessionHero01]

Safe guard a few places so exception cases be handled:

1. When deleting the space after a mention, the span will occupy the trailing of the text, which will have a special case of end offset == string length. Addition to it is definitely not going to go well.
2. When deleting the space after a mention, then start another mention, it will create a mention span overlap, which will also crash the app. The normalisation will also need to take this into consideration.