fix "removing parent element of a dragging node will throw an error". close #166

src/DragAndDrop.js

@@ -76,7 +76,10 @@
 
                 delete dd.node;
 
-                (layer || node).draw();
+                if (node.getLayer() || layer) {
+                  (layer || node).draw();
+                }
+
             }
         },
         _endDragAfter: function(evt) {

test/unit/DragAndDrop-test.js

@@ -286,4 +286,34 @@ suite('DragAndDrop', function() {
         });
 
     });
-});
+
+    // ======================================================
+    test('removing parent of draggable node should not throw error', function() {
+        var stage = addStage();
+        var layer = new Konva.Layer();
+        stage.add(layer);
+        var circle = new Konva.Circle({
+            x: 380,
+            y: stage.getHeight() / 2,
+            radius: 70,
+            strokeWidth: 4,
+            fill: 'red',
+            stroke: 'black',
+            draggable: true
+        });
+
+        layer.add(circle);
+        stage.simulateMouseMove({
+            x: stage.width() / 2 + 5,
+            y: stage.height() / 2
+        });
+
+        circle.startDrag();
+        try {
+          layer.destroy();
+          assert.equal(true, true, 'no error, that is very good');
+        } catch (e) {
+          assert.equal(true, false, 'error happened');
+        }
+    });
+});