BLE RX: fix for empty text prompt crash

[gullradriel]

This is a PR to fix #2462

Problem was that putting empty text in text_prompt for save entry or list filename was first changing the theme by writing bad things in memory, and a second call with the same empty behavior was crashing the app.

I didn't find any nice and clever way to do it. I did it my way.

At least that patch is solving the issue and displaying correct error messages.

(There are two other error message that can be triggered, but I didn't took screenshot of them)

PS: if one is asking himself why I didn't use class functions and all instead of the static variable, it's because BLE RX is using 3 differents classes and when I tried it was crashing because of navigation mayhem (nav pop was not clearing correctly, crashing, etc)

[htotoo]

Later we may need to get another solution for the "if" in the "on_timer()", but good for now, as it works as intended.