Properly handle right clicks without selectd items

ToeCracker/MainForm.cs

@@ -89,9 +89,11 @@ namespace ToeCracker {
                     return null;
 
                 string word = (string)((ListBox)this.tabControl.SelectedTab.Controls[0]).SelectedItem;
-                word = word.Trim();
-                
+                if (word == null)
+                    return null;
+
                 // Ignore empty lines and subgroups. Subgroups are represented as "-- ANTONYMS --"
+                word = word.Trim();
                 if (word == String.Empty || word.StartsWith("-- "))
                     return null;