add nullptr check to createTable() method as done for CSCs, MDTs and TGCs
this MR adds a nullptr check to the
createTable() method of the
sTGC_RegionSelectorTable.cxx as it is also done for the CSCs, MDTs and TGCs already.
The nullptr check can become important in case an identifier dictionnary in the
IdDictParser package contains identifiers to detector elements which are not part of a layout. Without the nullptr check, the code would cause a segmentation violation.