Fix 'Uncaught TypeError: Cannot read property 'value' of undefined' #6556 (#6574)
Fix 'Uncaught TypeError: Cannot read property 'value' of undefined' #6556 (#6574)

closes #6556
airbnb/superset,apache/incubator-superset,airbnb/caravel,zhouyao1994/incubator-superset,airbnb/superset,mistercrunch/panoramix,apache/incubator-superset,apache/incubator-superset,airbnb/caravel,mistercrunch/panoramix,airbnb/superset,apache/incubator-superset,zhouyao1994/incubator-superset,zhouyao1994/incubator-superset,zhouyao1994/incubator-superset,airbnb/superset,mistercrunch/panoramix,airbnb/caravel,zhouyao1994/incubator-superset,apache/incubator-superset,mistercrunch/panoramix,airbnb/caravel