If the action is stateful (e.g. created with
g_simple_action_new_stateful()) then this function returns the
#GVariantType of the state. This is the type of the initial value
given as the state. All calls to g_action_change_state() must give a
#GVariant of this type and g_action_get_state() will return a
#GVariant of the same type.
If the action is not stateful (e.g. created with g_simple_action_new())
then this function will return %NULL. In that case, g_action_get_state()
will return %NULL and you must not call g_action_change_state().
Queries the type of the state of @action.
If the action is stateful (e.g. created with g_simple_action_new_stateful()) then this function returns the #GVariantType of the state. This is the type of the initial value given as the state. All calls to g_action_change_state() must give a #GVariant of this type and g_action_get_state() will return a #GVariant of the same type.
If the action is not stateful (e.g. created with g_simple_action_new()) then this function will return %NULL. In that case, g_action_get_state() will return %NULL and you must not call g_action_change_state().
Return: the state type, if the action is stateful