fix(core/GrahicsFactory): account for removed shapes on updateContainments

Related to bpmn-io/bpmn-js#2269

Author: jarekdanielak

lib/core/GraphicsFactory.js

@@ -192,6 +192,11 @@ GraphicsFactory.prototype.updateContainments = function(elements) {
     forEach(children.slice().reverse(), function(child) {
       var childGfx = elementRegistry.getGraphics(child);
 
+      // shape was removed
+      if (!childGfx) {
+        return;
+      }
+
       prependTo(childGfx.parentNode, childrenGfx);
     });
   });