Genie:  Remove bogus assertion in destroy_layer()

This is causing a crash on exit if a stack view is used.  Instead, check
for null and bail out.
Genie:  Tweak stack_remove() to simplify patch
Genie:  RemoveAllViewParameters() for stack layers

Otherwise, stack layer subviews don't get reaped, and won't be destroyed
until after main() exits.
Merge branch 'stack-layer-cleanup-fix'