feat(Viewer): make importXML callback optional

People could be hooking up with import via events, too.

lib/Modeler.js

@@ -117,7 +117,7 @@ module.exports = Modeler;
 /**
  * Create a new diagram to start modeling.
  *
- * @param {Function} done
+ * @param {Function} [done]
  */
 Modeler.prototype.createDiagram = function(done) {
   return this.importXML(initialDiagram, done);

lib/Viewer.js

@@ -144,10 +144,13 @@ module.exports = Viewer;
  * You can use these events to hook into the life-cycle.
  *
  * @param {String} xml the BPMN 2.0 xml
- * @param {Function} done invoked with (err, warnings=[])
+ * @param {Function} [done] invoked with (err, warnings=[])
  */
 Viewer.prototype.importXML = function(xml, done) {
 
+  // done is optional
+  done = done || function() {};
+
   var self = this;
 
   // hook in pre-parse listeners +

test/spec/ViewerSpec.js

@@ -769,6 +769,23 @@ describe('Viewer', function() {
       });
     });
 
+
+    it('should work without callback', function(done) {
+
+      // given
+      var viewer = new Viewer({ container: container });
+
+      var xml = require('../fixtures/bpmn/simple.bpmn');
+
+      // when
+      viewer.importXML(xml);
+
+      // then
+      viewer.on('import.done', function(event) {
+        done();
+      });
+    });
+
   });