Ensure canvas copy texscreen will not crash if render target is configured without copy buffers. Closes #24749.

drivers/gles3/rasterizer_canvas_gles3.cpp

@@ -1144,6 +1144,11 @@ void RasterizerCanvasGLES3::_canvas_item_render_commands(Item *p_item, Item *cur
 
 void RasterizerCanvasGLES3::_copy_texscreen(const Rect2 &p_rect) {
 
+	if (storage->frame.current_rt->effects.mip_maps[0].sizes.size() == 0) {
+		ERR_EXPLAIN("Can't use screen texture copying in a render target configured without copy buffers");
+		ERR_FAIL();
+	}
+
 	glDisable(GL_BLEND);
 
 	state.canvas_texscreen_used = true;