apply / pop only work with a stash (not a branch). they merge the stash to your current work. the apply / pop can also create a new branch
when you checkout a branch with un-commited changes (which you are doing), git will stash them for you (push a stash). if you don't want this new stash, just drop it.
your other option is cleanup uncommitted changes before the checkout of the branch you want.
if you are trying to apply a stash to known branch. checkout the branch first, then apply / pop the stash.