Chat: Bug report in chat module on loading, unloading and reloading

When we reload or unload and load again the chat module it throws a gtk error and segmentation fault

`(mavproxy.py:13354): Gtk-CRITICAL **: 00:08:18.468: gtk_text_attributes_ref: assertion 'values != NULL' failed`

`(mavproxy.py:13354): Gtk-CRITICAL **: 00:08:18.468: gtk_text_attributes_ref: assertion 'values != NULL' failed
Segmentation fault (core dumped)`

To reproduce this bug
Start mavproxy.py
module load chat
module unload chat
module load chat

or

Start mavproxy.py
module load chat
module reload chat

Reproduce the crash with mavproxy.py using `module load chat`, then `module unload chat` followed by `module load chat`, or `module reload chat`. Start by locating the chat module entry point and tracing its loading and unloading lifecycle; done means repeated reloads no longer emit the GTK assertion or cause a segmentation fault.

Written by the indexing model from the issue text.