restore clone return type

src/Meshes/mesh.ts

@@ -2201,7 +2201,7 @@ export class Mesh extends AbstractMesh implements IGetSetVerticesData {
      * @param clonePhysicsImpostor allows/denies the cloning in the same time of the original mesh `body` used by the physics engine, if any (default `true`)
      * @returns a new mesh
      */
-    public clone(name: string = "", newParent: Nullable<Node> = null, doNotCloneChildren?: boolean, clonePhysicsImpostor: boolean = true): Nullable<AbstractMesh> {
+    public clone(name: string = "", newParent: Nullable<Node> = null, doNotCloneChildren?: boolean, clonePhysicsImpostor: boolean = true): Mesh {
         return new Mesh(name, this.getScene(), newParent, this, doNotCloneChildren, clonePhysicsImpostor);
     }