Crash when saving after adding black mask to layer

[nfaltermeier]

ArmorPaint version:

e1d91d5c65b4914c60d46dba60f1a3db874aa25f

OS / GPU model: Windows 10 / RX 7900 XTX

Issue description: The program crashes when trying to save an .arm after adding a black mask to a layer

Steps to reproduce:

1. Add a new black mask to layer 1 of the default cube project
2. File > Save as and save the file