There is a Q_ASSERT in KisVisualColorSelector::slotRebuildSelectors() that triggers if you try to load (open or create) a grayscale/alpha image. Q_ASSERT(m_d->widgetlist.size() == 2); Only happens on the 3.2 branch for me.
Can you also attach the full backtrace?
Looking at the code this should be 100% reproducible, maybe depending on what widget is visible. I looked through the code a bit more than that assert shouldn't really be there, and no code path exists where removing it is a problem. It's also removed on the main branch. I fixed it by removing it from the 3.2 branch in 2c28382d84ccc7de5452326ed672d0fdb699efb1 but forgot to tag this bug. Marking this as resolved.