fix(viewer): do not clear diagram if no diagram to clear

Closes #1181

lib/Viewer.js

@@ -479,6 +479,11 @@ Viewer.prototype.getModules = function() {
  * @method Viewer#clear
  */
 Viewer.prototype.clear = function() {
+  if (!this.getDefinitions()) {
+
+    // no diagram to clear
+    return;
+  }
 
   // remove businessObject#di binding
   //

test/spec/ViewerSpec.js

@@ -1614,6 +1614,25 @@ describe('Viewer', function() {
 
   describe('#clear', function() {
 
+    it('should NOT clear if no diagram', function() {
+
+      // given
+      var viewer = new Viewer({ container: container });
+
+      var eventBus = viewer.get('eventBus');
+
+      var spy = sinon.spy();
+
+      eventBus.on('diagram.clear', spy);
+
+      // when
+      viewer.clear();
+
+      // then
+      expect(spy).not.to.have.been.called;
+    });
+
+
     it('should not throw if diagram is already empty', function() {
 
       // given