When pick two CFs to define threads in POV, it has trouble creating distance matrix for clustering #86

It happens on the Deidentified_Derm_25_Visits.csv when I pick visit_ID and workstation_ID, but it does NOT happen on the Derm-AC2-Feb-1day.csv. Strange.

Here is the error:
Warning: Error in distances: At iterators.c:759 : Cannot create iterator, invalid vertex id, Invalid vertex id
Stack trace (innermost first):
101: distances
100: dist_matrix_network [ThreadNet_Core.R#737]
99: clusterEvents [ThreadNet_Core.R#656]
98: eval [ThreadNet_Core.R#323]
97: eval
96: withProgress
95: ThreadOccByPOV [ThreadNet_Core.R#205]
94: reactive:threadedOcc [/Users/btp/Documents/RStuff/ThreadNet3/ThreadNet/R/server.R#104]
83: threadedOcc
82: exprFunc [server/choosePOV.R#57]
81: widgetFunc
80: func
79: origRenderFunc
78: renderFunc
77: origRenderFunc
76: output$Thread_Tab_Output_1
1: runApp