Same behaviour using polylines ("Add lines and polygons to symbol body" tool).
To reproduce it:
-add a polyline
-select it and drag a point
-continue dragging the point around until the "assert".
Tested on commit 2aea26de1e38eadf868fead7e89d92913885fbe3
Dragging the control point of an arc does not show this assert (but another strange behaviour: it moves the little squares that should be the start and the end point of the arc, but the arc is not redrawn/changed), but this seems a bug unrelated to this one).
Same behaviour using polylines ("Add lines and polygons to symbol body" tool).
To reproduce it:
-add a polyline
-select it and drag a point
-continue dragging the point around until the "assert".
Tested on commit 2aea26de1e38ead f868fead7e89d92 913885fbe3
Dragging the control point of an arc does not show this assert (but another strange behaviour: it moves the little squares that should be the start and the end point of the arc, but the arc is not redrawn/changed), but this seems a bug unrelated to this one).