Problem with joint genotyping in gatk 3.7

Hi,

When the "GenotypeGVCFs" module in GATK 3.7 is used to generate joint calling it gives either one of these two errors - null pointer exception or a concurrent modification exception. However, on repeatedly running the same samples it some times completes the joint genotyping successfully (no errors). Attached are details on the errors, please let us know how to address this issue.

Thanks!