Found a work around my self.
In case of an empty track after copying it from an other project, select and erase the empty wave file of the track. Go to media/trash. Find the deleted wave file and drag the file into the track. After this, the wave file is visible again and shows the original audio data. When one switches projects after this, no empty tracks occurs anymore.
It helps when the wave file name is changed directly after copying the wave file from an other project. This helps finding the file in the trash after deleting it.