MSTransferor crashing to check the final pileup location - testbed
**Impact of the bug**
ReqMgr2MS - MSTransferor

**Describe the bug**
The service is failing the cycle because it's accessing an unbound variable. Traceback will be provided below.

**How to reproduce it**
none

**Expected behavior**
MSTransferor should be able to check the final location of pileup samples ;)

**Additional context and error message**
Traceback from vocms0731:
```
2020-09-12 02:29:11,629:INFO:MSTransferor: Finding final pileup destination for request: wmagent_SC_MultiPU_khurtado_newsubmit_SC_MultiPU_200909_180630_7065
2020-09-12 02:29:11,631:ERROR:MSManager: MS daemon error: local variable 'commonPsns' referenced before assignment
Traceback (most recent call last):
  File "/data/srv/beHG2009h/sw/slc7_amd64_gcc630/cms/reqmgr2ms/0.4.0.pre8/lib/python2.7/site-packages/WMCore/MicroService/Unified/MSManager.py", line 43, in daemon
    func(reqStatus)
  File "/data/srv/beHG2009h/sw/slc7_amd64_gcc630/cms/reqmgr2ms/0.4.0.pre8/lib/python2.7/site-packages/WMCore/MicroService/Unified/MSManager.py", line 141, in transferor
    res = self.msTransferor.execute(reqStatus)
  File "/data/srv/beHG2009h/sw/slc7_amd64_gcc630/cms/reqmgr2ms/0.4.0.pre8/lib/python2.7/site-packages/WMCore/MicroService/Unified/MSTransferor.py", line 211, in execute
    self.checkPUDataLocation(wflow)
  File "/data/srv/beHG2009h/sw/slc7_amd64_gcc630/cms/reqmgr2ms/0.4.0.pre8/lib/python2.7/site-packages/WMCore/MicroService/Unified/MSTransferor.py", line 515, in checkPUDataLocation
    pnns = self._findFinalPULocation(wflow)
  File "/data/srv/beHG2009h/sw/slc7_amd64_gcc630/cms/reqmgr2ms/0.4.0.pre8/lib/python2.7/site-packages/WMCore/MicroService/Unified/MSTransferor.py", line 561, in _findFinalPULocation
    if not commonPsns:
UnboundLocalError: local variable 'commonPsns' referenced before assignment
[12/Sep/2020:02:29:22] vocms0731.cern.ch 127.0.0.1 "GET /ms-transferor/data
```
diff --git a/src/python/WMCore/MicroService/Unified/MSTransferor.py b/src/python/WMCore/MicroService/Unified/MSTransferor.py
index 4097ae663..d7005fa78 100644
--- a/src/python/WMCore/MicroService/Unified/MSTransferor.py
+++ b/src/python/WMCore/MicroService/Unified/MSTransferor.py
@@ -534,6 +534,7 @@ class MSTransferor(MSCore):
                 dsetName = dataIn["name"]
                 campConfig = self.campaigns[dataIn['campaign']]
 
+                commonPsns = set()
                 # if the dataset has a location list, use solely that one
                 if campConfig['Secondaries'].get(dsetName, []):
                     commonPsns = set(psns) & set(campConfig['Secondaries'][dsetName])
['src/python/WMCore/MicroService/Unified/MSTransferor.py']