RVIZ showing white block when put a 'floating joint' in robot's urdf

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Hi all, I'm making a small robot for learning ROS and while making its URDF file I can't really connect the 'world' frame to the 'base_link' frame with a 'floating joint'. If I do so, RVIZ makes the complete robot a white block and in the 'Display panel' of RVIZ, the Transformation of many joints gets messed. There's nothing much out there talking about 'floating' joint and hence this question. I will appreciate all your answers. Thanks.