Fix a crash in Zenmap Compare Results.

CHANGELOG

@@ -1,5 +1,9 @@
 # Nmap Changelog ($Id$); -*-text-*-
 
+o Fix a crash in Zenmap when using Compare Results:
+  AttributeError: 'NoneType' object has no attribute 'get_nmap_output'
+  [Daniel Miller]
+
 o [NSE] Fix http.get_url function when used with https scheme. Previously,
   plaintext http to port 443 was attempted first. [jah]
 

zenmap/zenmapGUI/DiffCompare.py

@@ -302,7 +302,10 @@ class ScanChooser(HIGVBox):
     def get_nmap_output(self):
         """Return the currently selected scan's output as a string, or None if
         no valid scan is selected."""
+        if self.parsed_scan is not None:
             return self.parsed_scan.get_nmap_output()
+        else:
+            return None
 
     nmap_output = property(get_nmap_output)
     parsed_scan = property(get_parsed_scan)