WebGLBackground: Reset meshes in `dispose()`. (#30257)

src/renderers/webgl/WebGLBackground.js

@@ -249,6 +249,8 @@ function WebGLBackground( renderer, cubemaps, cubeuvmaps, state, objects, alpha,
 			boxMesh.geometry.dispose();
 			boxMesh.material.dispose();
 
+			boxMesh = undefined;
+
 		}
 
 		if ( planeMesh !== undefined ) {
@@ -256,6 +258,8 @@ function WebGLBackground( renderer, cubemaps, cubeuvmaps, state, objects, alpha,
 			planeMesh.geometry.dispose();
 			planeMesh.material.dispose();
 
+			planeMesh = undefined;
+
 		}
 
 	}