This affects tablets that are not listed in libwacom, specifically, tablets where libwacom does not provide the stylus axis information.
On tablet init, we check the event codes. A tablet may have ABS_Z and thus have rotation. Or it has BTN_TOOL_MOUSE + ABS_TILT_X/Y and thus have rotation. On init, all axes are marked as 'changed' to ensure we send the data in the first event. For non-libwacom tablets, the tool has the same bits set as the tablet provides, so if mouse+tilt is there, we have rotation on all tools. But the code itself relies on ABS_Z for rotation on all the styli (except mouse/lense).
In the case of Aiptek tablets we do have mouse+tilt but we *don't* have ABS_Z. If the first tool that comes into proximity is *not* the mouse/lens tool , we try to update the rotation by accessing ABS_Z. This leads to a null-pointer dereference in tablet_update_artpen_rotation() on the very first event.
It doesn't matter that the tool never sends the actual events - because all axes are marked as 'changed' on proximity in, we always access the axis on the first event and crash.
 see also Bug 104911
And for extra fun, the series starting here:
Author: Peter Hutterer <email@example.com>
Date: Mon Feb 5 09:11:42 2018 +1000
tablet: don't set rotation on a tool if we don't have ABS_Z