Don't crash on "%TaskWindow -display -ctrl"
This long-standing bug means that
%TaskWindow -display -ctrl
without an editor already running causes a branch through zero.
The bug is reported here https://www.riscosopen.org/forum/forums/4/topics/3627
This manifests also in errors when using a task window with Zap, see https://www.riscosopen.org/forum/forums/11/topics/16002