Merge pull request phaserjs#4118 from pierpo/patch-1

Fix issue with null config in Arcade PhysicsGroup constructor

Author: photonstorm

src/physics/arcade/PhysicsGroup.js

@@ -121,6 +121,14 @@ var PhysicsGroup = new Class({
                 singleConfig.removeCallback = this.removeCallbackHandler;
             });
         }
+        else
+        {
+            // config is not defined and children is not a plain object nor an array of plain objects
+            config = {
+                createCallback: this.createCallbackHandler,
+                removeCallback: this.removeCallbackHandler
+            }
+        }
 
         /**
          * The physics simulation.