I have tested Paint Bucket tool intensely, but can't crash Pinta. But I am sure crashes did appear in the past versions, I think I could always reproduce when canvas was resized and then paint bucket used.
This was probably unexpectedly fixed in one of the several fixes lately.
Nikki, if you can test and write step by steps how to reproduce a problem. Also please write Pinta version (Help | About menu) and what is your operating system and version.
Can you provide more details on the full error message and which version of Pinta you're using? As well as any steps taken before using the fill tool