Custom expandable class issue

Hello all, I dont really know what to do I made my custom class RangeCondition and I want it to be expandable in PropertyGrid in Parameters window. I think I done it. But in a LOG I have the same error after compiling and refresh it (just for the first time after compile). And this property doesnt remeber its value Would you be so king and help me somehow ??

Code:
[TypeConverterAttribute(typeof(ExpandableObjectConverter))]
[Serializable()]
public class RangeCondition: ISerializable
{
private OF_OperatorType condoperator = OF_OperatorType.NoOperator; // enumerator defined above
private OF_ConditionType condtype = OF_ConditionType.OpositeSideDiagonally; // enumerator defined above
private double multiplier = 1;
private int increment = 0;
private int condvalue = 0;
public NinjaTrader.Indicator.Indicator Reference = null;


public RangeCondition() {
Operator = OF_OperatorType.NoOperator;
Type = OF_ConditionType.OpositeSideDiagonally;
Multiplier = 1;
Increment = 0;
Value = 0;
}


public override string ToString()
{
string rtn = "no condition";
if (condoperator!=OF_OperatorType.NoOperator) {
rtn = "Number ";
} else {
return rtn;
}

if (condoperator==OF_OperatorType.GreaterThan) rtn+= "> ";
if (condoperator==OF_OperatorType.GreaterThanOrEqualTo) rtn+= ">= ";
if (condoperator==OF_OperatorType.LessThan) rtn+= "< ";
if (condoperator==OF_OperatorType.LessThanOrEqualTo) rtn+= "<= ";

if (condtype==OF_ConditionType.OpositeSide) rtn+= "OS";
if (condtype==OF_ConditionType.AvgTradesPerPeriod) rtn+= "AvgTPP";
if (condtype==OF_ConditionType.Value) rtn+= condvalue+"";

if (Multiplier>1) rtn+= "*"+Multiplier;
if (Increment>0) rtn+= "+"+Increment;

return rtn;
}


public void GetObjectData(SerializationInfo serInfo, StreamingContext context)
{
serInfo.AddValue("Operator",Operator);
serInfo.AddValue("Type",Type);
serInfo.AddValue("Multiplier",Multiplier);
serInfo.AddValue("Increment",Increment);
serInfo.AddValue("Value",Value);
}

private RangeCondition(SerializationInfo serInfo,StreamingContext context)
{
this.Operator = (OF_OperatorType)Enum.Parse(typeof(OF_OperatorType), serInfo.GetString("Operator"));
this.Type = (OF_ConditionType)Enum.Parse(typeof(OF_ConditionType), serInfo.GetString("Type"));
this.Value = serInfo.GetInt32("Value");
this.Increment = serInfo.GetInt32("Increment");
this.Multiplier = serInfo.GetDouble("Multiplier");
}
.
.
.
and so on
}

Error text:
Failed to set property 'R1_Condition1' for indicator 'TestIndicator':
Object of type RangeCondition cannot be converted to type RangeCondition.


Is it a problem of serialization or WHAT ???
I would appreciate any help.
Thank you very much.

RS