fix mesh cloning when scene has no getPhysicsEngine function

src/Meshes/mesh.ts

@@ -400,11 +400,13 @@ export class Mesh extends AbstractMesh implements IGetSetVerticesData {
             }
 
             // Physics clone
-            var physicsEngine = this.getScene().getPhysicsEngine();
-            if (clonePhysicsImpostor && physicsEngine) {
-                var impostor = physicsEngine.getImpostorForPhysicsObject(source);
-                if (impostor) {
-                    this.physicsImpostor = impostor.clone(this);
+            if (scene.getPhysicsEngine) {
+                var physicsEngine = scene.getPhysicsEngine();
+                if (clonePhysicsImpostor && physicsEngine) {
+                    var impostor = physicsEngine.getImpostorForPhysicsObject(source);
+                    if (impostor) {
+                        this.physicsImpostor = impostor.clone(this);
+                    }
                 }
             }