Stunner - exception when setting ruleflow group to a business rule task

When I try to set ruleflow group to a business rules task in Stunner (see the screenshot), I get the following exception:

org.kie.workbench.KIEWebapp-0.js:63950 Error caused by: IllegalArgumentException: Duplicate value:
at HDm.fN [as oe] (org.kie.workbench.KIEWebapp-0.js:63869:20106)
at HDm.iN [as qe] (org.kie.workbench.KIEWebapp-0.js:63869:20312)
at HDm.dN (org.kie.workbench.KIEWebapp-0.js:38391:48)
at HDm.lN (org.kie.workbench.KIEWebapp-0.js:22107:19)
at HDm.s$ (org.kie.workbench.KIEWebapp-0.js:22131:19)
at HDm (org.kie.workbench.KIEWebapp-0.js:22425:20)
at tnm (org.kie.workbench.KIEWebapp-0.js:54292:61)
at unm (org.kie.workbench.KIEWebapp-0.js:54002:132)
at WAU (org.kie.workbench.KIEWebapp-0.js:21379:19)
at RAU (org.kie.workbench.KIEWebapp-0.js:46471:48)
at MAU (org.kie.workbench.KIEWebapp-0.js:62250:512)
at eBU.fBU [as Lk] (org.kie.workbench.KIEWebapp-0.js:64113:2133)
at b0b.c0b [as Nb] (org.kie.workbench.KIEWebapp-0.js:63874:10503)
at s0b (org.kie.workbench.KIEWebapp-0.js:60853:212)
at j0b (org.kie.workbench.KIEWebapp-0.js:57398:60)
at B0 (org.kie.workbench.KIEWebapp-0.js:31693:26)
at OAw.O0 [as Wc] (org.kie.workbench.KIEWebapp-0.js:63870:12990)
at e0b (org.kie.workbench.KIEWebapp-0.js:48341:90)
at wnm (org.kie.workbench.KIEWebapp-0.js:49844:100)
at Dnm.Enm [as wk] (org.kie.workbench.KIEWebapp-0.js:63880:24787)
at VYb.WYb [as Nb] (org.kie.workbench.KIEWebapp-0.js:63874:5051)
at s0b (org.kie.workbench.KIEWebapp-0.js:60853:212)
at j0b (org.kie.workbench.KIEWebapp-0.js:57398:60)
at B0 (org.kie.workbench.KIEWebapp-0.js:31693:26)
at evt.O0 [as Wc] (org.kie.workbench.KIEWebapp-0.js:63870:12990)
at MYb (org.kie.workbench.KIEWebapp-0.js:55485:138)
at D0 (org.kie.workbench.KIEWebapp-0.js:52863:115)
at evt.T0 [as Xf] (org.kie.workbench.KIEWebapp-0.js:63870:13156)
at OAw.QAl [as Xf] (org.kie.workbench.KIEWebapp-0.js:63877:14504)
at jXl (org.kie.workbench.KIEWebapp-0.js:48342:85)
at HTMLSelectElement.YYl (org.kie.workbench.KIEWebapp-0.js:45879:46)
at qKb (org.kie.workbench.KIEWebapp-0.js:35598:30)
at tKb (org.kie.workbench.KIEWebapp-0.js:57021:60)
at HTMLSelectElement.eval (org.kie.workbench.KIEWebapp-0.js:45272:48)