[Bug] [RTK Facet mosaic] paintSIVIcon causes crash during survey-in

[PaulZC]

I'm seeing a crash during Base survey-in with firmware 2.1:

[PaulZC]

It looks like this is caused by this commit:

fb8b5df

There's a weakness in the logic which allows icon to remain nullptr.

[PaulZC]

I've fixed the crash, but now I see:

[PaulZC]

This is resolved in PR #561