the gms1 version has a mistake in it
instance_destroy() has to be called as a function with no arguments
so id change it to this
if(myTextbox != noone){
with myTextbox{
instance_destroy()};
myTextbox = noone;
Actually this isn't a mistake; the function has an optional argument. Its default is just to destroy the calling instance, but you can destroy a specific instance other than the calling one via the optional argument. This is according to the GameMaker documentation on the function.
As far as I know this has never caused me any problems -- have you encountered some errors?