Abnormal Termination

I am trying to modify "Skyliner" submitted bycaddisfly0412.
The whole piece is in cut time except the last 2 bars.
I place a double bar after bar 156 and then select the next bar and insert time signature 4/4 and a semibreve.
Which results in

.
If I try to do anything with the final bar Musescore terminates.
Any ideas?

Thanks, Adrian Pollard