Reproduce: Select an area, do any transformation on it. Select brush tool. When escaping from transformation mode one usually selects the brush tool. However krita crashes without any prior warning (no RAM bootleneck...)
Are you really using 3.0? If so, could you check the latest release, 3.1.2. Which distribution of Linux are you using?
Please send us the relevant information.
I can't reproduce this, neither in Krita 4.1.0-pre-alpha nor in 3.3.3 version - Ubuntu 14.04 Unity. (I'm going to mark this report as RESOLVED).