added handling for non-selection in boundingbox extension

extensions/fablabchemnitz/boundingbox.py

@@ -39,8 +39,13 @@ class DrawBBoxes(inkex.Effect):
                 for id, item in self.svg.selected.items():
                     self.drawBBox(item.bounding_box())
             else:
+                inkex.utils.debug("")
                 #self.drawBBox(self.svg.get_selected_bbox()) #works for InkScape (1:1.0+devel+202008292235+eff2292935) @ Linux and for Windows (but with deprecation)
                 self.drawBBox(self.svg.selection.bounding_box()) #works for InkScape 1.1dev (9b1fc87, 2020-08-27)) @ Windows
 
+        else:
+            inkex.errormsg('Please select some objects first.')
+            return
+
 if __name__ == '__main__':
     DrawBBoxes().run()