<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def create_shared(self, name, ref):
 """ For the app backends to create the GLShared object. Parameters name : str The name. ref : object The reference. """

        if self._shared is not None:
            raise RuntimeError('Can only set_shared once.')
        self._shared = GLShared(name, ref)