The text should be parented to the null. Open the positing parameters of the null X,Y,Z and copy/paste this to the text layer’s X,Y,Z to locate it exactly where the null is in 3D space. If you don’t see Z axis available on your text then it needs to be converted to 3D text first before copying the parameters. Once you locate the text to the same location of the null and parent it, you can delete the keyframes for the X,Y,Z on the text layer. It will use the Null’s positioning at that point. The null also needs to be 3D but it already probably is from the track.
Thanks Chris