Fix text tool producing "null" when cancel button pressed

src/js/drawtools.js

@@ -49,7 +49,7 @@ function activateTextTool() {
         return;
     }
     var text = prompt("Enter a line of text to add to the document:");
-    if (text == "") {
+    if (text == "" || text == null) {
         return;
     }
     var canvas = document.createElement('canvas');