Fixed weird bug in which menu entries ( like, "New->Entity Relationship Diagram", "New->Component Diagram" etc ) used to disappear suddenly.

Scenario in which this used to happen:
1. Right Click on the folder "Use Case View" -> "New" -> "Use Case Diagram"
2. Right Click on the folder "Component View" -> "New" -> "Component Diagram"
3. Right Click on the folder "Use Case View"-> "New" -> ... ( Where did the entry "Use Case Diagram" disappear ???? )

The action used to be deleted in the destructor of ListPopupMenu :O ( as it used to maintain a local copy of the pointer to the action ).