The problem here is not with the animated line plotting it is what you post at the top - that your axes no longer exist. That is why it creates a new axes when you don't specify one and why it throws an error when you do.
Judging by the name of your axes I assume they are axes defined in GUIDE? If so I'm not sure how you would end up deleting the axes. You should search through your code for any instruction that involves handles.axes1 and check that you are not explicitly deleting the axes. Axes in a GUIDE GUI have the same life time as the GUI itself unless you explicitly delete them.
I notice though also that your callback does not include a
guidata( hObject, handles )
call so any changes you make to the handles structure there will be lost after the callback. That in itself is not the cause of this particular problem, but it is a hint that you might have done something incorrect with the handles structure previously also.
