Fix redundant insertion marker after dragging

Signed-off-by: Songlin Jiang <songlin.jiang@csc.fi>

src/multiselect_block_dragger.js

@@ -31,6 +31,16 @@ export class MultiselectBlockDragger extends Blockly.BlockDragger {
     this.blockSelection = blockSelectionWeakMap.get(workspace);
   }
 
+  /**
+   * Dispose of this block dragger.
+   */
+  dispose() {
+    super.dispose();
+    this.blockDraggers_.forEach((blockDragger) => {
+      blockDragger.dispose();
+    });
+  }
+
   /**
    * Prepares the block dragger for a new drag.
    * @param {!Blockly.utils.Coordinate} currentDragDeltaXY How far the pointer
@@ -180,7 +190,7 @@ export class MultiselectBlockDragger extends Blockly.BlockDragger {
          */
         this.draggingBlock_.getDescendants(false).forEach(function(block) {
           if (!block.getChildren().length) {
-            block.render();
+            block.queueRender();
           }
         });