Close session panels before setting worldEditor to None

Panels expect worldEditor to not be None and this was spamming exceptions from one of the list models.

src/mcedit2/editorsession.py

@@ -824,12 +824,13 @@ class EditorSession(QtCore.QObject):
             if ret == QtGui.QMessageBox.Cancel:
                 return False
 
+        for panel in self.panels:
+            panel.close()
+
         self.editorTab.saveState()
         self.worldEditor.close()
         self.worldEditor = None
 
-        for panel in self.panels:
-            panel.close()
         return True
 
     # --- Inspector ---