Fix bug with mesh related commands when no mesh is selected

The select_mesh method did not return a tuple on every outcome. So it
failed when nothing was selected.

fixes #25

lithophane_utils.py

@@ -40,14 +40,14 @@ def findSelectedMesh():
   selection = FreeCADGui.Selection.getSelection()
 
   if len(selection) != 1:
-    return None
+    return (None, None)
 
   selectedObject = selection[0]
 
   if not hasattr(selectedObject, 'Mesh'):
-    return None
+    return (None, None)
 
-  return selection[0].Mesh, selection[0].Label
+  return (selection[0].Mesh, selection[0].Label)
 
 def vectorAtGround(vector):
   return FreeCAD.Vector(vector.x, vector.y, 0)

measure.py

@@ -15,7 +15,7 @@ def GetResources(self):
                 'Pixmap': iconPath('Measure.svg')}
 
     def Activated(self):
-        mesh = lithophane_utils.findSelectedMesh()[0]
+        mesh, label = lithophane_utils.findSelectedMesh()
 
         if mesh is None:
           qtutils.showInfo("No Mesh selected", "Select exactly one Mesh to continue")