Diff menu item enabled when "Sites" tree root node and a child node are selected

[GoogleCodeExporter]

Steps to reproduce the issue:
1. Run ZAP and generate a message;
2. Select the "Sites" tree root node and a child node (the generated message);
3. Right click in the tree to show the pop up menu;
5. Note that the menu item "Diff 2 responses" is enabled when it shouldn't, if 
selected two (equal) NullPointerException are thrown:
[...]
ERROR org.zaproxy.zap.view.PopupMenuHistoryReference  - 
java.lang.NullPointerException
    at org.zaproxy.zap.extension.diff.PopupMenuDiff.performActions(Unknown Source)
    at org.zaproxy.zap.view.PopupMenuHistoryReference$1.actionPerformed(PopupMenuHistoryReference.java:100)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
[...]

Remarks:
The pop up menu item is being called twice.

Add-on version:
"Diff" version 1.

Original issue reported on code.google.com by THC...@gmail.com on 4 Mar 2013 at 5:00

[GoogleCodeExporter]

https://code.google.com/p/zap-extensions/source/detail?r=390

Original comment by THC...@gmail.com on 4 Mar 2013 at 5:48

• Changed state: Committed

[GoogleCodeExporter]

Original comment by psii...@gmail.com on 4 Mar 2013 at 9:18

• Added labels: Build-20130304

[GoogleCodeExporter]

Fixed in 2.1.0

Original comment by psii...@gmail.com on 18 Apr 2013 at 9:49

• Changed state: Fixed