1. Create a new image (32x32)
2. Paint all pixels with full white
3. Export image as PNG
4. Create (or duplicate) a brush
5. Import the previously exported PNG as the shape of the brush
6. BAM. Pixelmator crashes.
It's weird to have a brush that is completely transparent (white in brush shape means 0 alpha). But it should not crash the app. Most likely people will make this same mistake of assuming white is opaque mask, and they may lose unsaved progress.
macOS High Sierra 10.13.6
MBP 15" 2018 - 2.6Ghz i7 - 32GB Ram - Radeon Pro 560X
Pixelmator 1.1.3 Monsoon
Is something not working like it should? Let us know.