Fix MCS exposures without frameId arg

To the command mcs expose object expTime=0.8 (without passing in visit= we get:
text="command failed: TypeError(\"unsupported operand type(s) for //: 'NoneType' and 'int'\") in _constructHeader() at /software/mhs/products/Linux64/ics_mcsActor/1.9.10a/python/mcsActor/Commands/McsCmd.py:452"

For that, frameId needs to be set in getNextFileIds after one is fetched from gen2.