Fixes #3712 - Core worflow perform action "select" should not make multiple values selectable.

app/assets/javascripts/app/controllers/_ui_element/_application_selector.coffee

@@ -485,8 +485,7 @@ class App.UiElement.ApplicationSelector
       if attribute.value && attribute.value[groupAndAttribute]
         config['value'] = @buildValueConfigValue(elementFull, elementRow, groupAndAttribute, elements, meta, attribute)
       if 'multiple' of config
-        config.multiple = true
+        config = @buildValueConfigMultiple(config, meta)
-        config.nulloption = false
       if config.relation is 'User'
         config.multiple = false
         config.nulloption = false
@@ -516,6 +515,11 @@ class App.UiElement.ApplicationSelector
     else
       elementRow.find('.js-value').removeClass('hide')
 
+  @buildValueConfigMultiple: (config, meta) ->
+    config.multiple = true
+    config.nulloption = false
+    return config
+
   @humanText: (condition) ->
     none = App.i18n.translateContent('No filter.')
     return [none] if _.isEmpty(condition)

app/assets/javascripts/app/controllers/_ui_element/core_workflow_perform.coffee

@@ -125,6 +125,14 @@ class App.UiElement.core_workflow_perform extends App.UiElement.ApplicationSelec
 
     super(elementFull, elementRow, groupAndAttribute, elements, meta, attribute)
 
+  @buildValueConfigMultiple: (config, meta) ->
+    if _.contains(['add_option', 'remove_option', 'set_fixed_to'], meta.operator)
+      config.multiple = true
+    else
+      config.multiple = false
+    config.nulloption = false
+    return config
+
   @HasPreCondition: ->
     return false