After I upgraded CodeEffects.Rule.Editor.Web.Core nuget from v5.0.14.6 to v5.0.19.4, I'm getting the following NullReferenceException when calling RuleEditor.GetClientSettings():
 Message: 
    System.NullReferenceException : Object reference not set to an instance of an object.
  Stack Trace: 
    Encoder.GetHashToken(MethodInfo m, String sourceTypeFullName)
    SourceLoader.GetTokenByMethod(XmlNode source, Type type, Type sourceObject, String methodName, XmlNodeList parameters, Boolean isMethod)
    SourceLoader.GetTokenBySourceMethodNode(XmlNode source, XmlNode method, Type sourceObject)
    Settings.Load(IControl control, ICollection`1 contextMenuRules, XmlDocument sourceXml, ICollection`1 dataSourceHolders, Boolean plainSourceXml)
    RuleEditor.GetConditions()
    RuleEditor.GetClientSettings()
Through elimination I zeroed in to the following method, which seems to be causing the error (this worked in v5.0.14.6):
[Method("IsSingle")]
public bool IsSingle<T>([Parameter(ValueInputType.Fields, Description = "array")] IEnumerable<T> array)
{
   return array != null && array.Distinct().SingleOrDefault() != null;
}