The problem you're having is a bug within Motocast. Once you sync your phone with a computer using Motocast it is linked with that computer, since it doesn't actually download any of the media to the device once you start the Music program it attempts to relink with the original computer to allow the streaming/playing of the songs you sync'd.
If you turn on your wifi (not clear if wifi is required), and turn on motocast on the original computer you can then reopen the Music app without difficulty. Then simply remove all folders (via the motocast application on your computer) that normally would sync via that computer.
Once you have finished removing the existance of folders/songs to sync to the phone, the Music app will bypass the Motocast startup process and ignore the empty Motocast slots and will no longer crash everytime you attempt to open it. Certainly beats resetting your phone.
