Fix bug that causes crash when nothing is double-clicked

EduNetworkBuilder/NetworkBuilder.cs

@@ -628,6 +628,7 @@ namespace EduNetworkBuilder
 
         private void pbNetworkView_Edit_Click(object sender, EventArgs e)
         {
+            if (ItemClickedOn == null) return;
             if (ItemClickedOn.GetNetType() == NetworkComponentType.microwave || ItemClickedOn.GetNetType() == NetworkComponentType.fluorescent)
                 return;
             if (ItemClickedOn != null)