With regards to the gdb[55]... editor tab that you are seeing, this message is generally erroneous. The debugger has at some point during the connection to the target tried to access memory at a point when the PC was not "valid", and then tried to display the source related to the "invalid" address - which has triggered the edit box with the"no source..." message in it.
Preventing this sporadically happening is proving hard, as it depends on all sorts of configuration and timings between the PC, the debug probe and the target. And many users won't actually see it in their particular setup.
Typically, the erroneous gdb[55]... tab can closed and you should just be able to click into the debug View (on the backtrace) to switch to the "real" current location in the editor view.
But we are continuing to look for a way of preventing this for a future IDE release.
Regards,
MCUXpresso IDE Support