Delete entire line with backspace key pressed down results in errors

[rogererens]

Problem When you delete an entire line of text with the backspace key held down, and 'hit' the block bullet in front of the line, I get this error message. It's another message than the one described in issue #1897 Also unlike that issue, clicking OK suffices to continue (i.e. no restart needed). When undoing and redoing multiple times, eventually another message pops up

Pressing the backspace key repeatedly (once for every character on the line to be deleted) works OK.

Screenshots/Demo

Athens Version 2.0.0.BETA 5