fixed get_step_form() checking for instance when it should be checking

for a class

xadmin/plugins/wizard.py

@@ -122,7 +122,7 @@ def get_step_form(self, step=None):
  return callback(self)
  elif hasattr(self.admin_view, str(callback)):
  return getattr(self.admin_view, str(callback))(self)
- elif isinstance(attrs, forms.BaseForm):
+ elif issubclass(attrs, forms.BaseForm):
  return attrs
  return None