Error on *._FieldSerializer ,the GWT's generated class - exception

Question

I write my own type of exception (extends from java Exception).
And I got exception on GWT's generated class xxx_FieldSerializer .
the error message look like this:
Object reference not set to an instance of an object.
com.xxx.shared.csharpsystem.exception.MyException_FieldSerializer Method name : instantiate LINE_NUMER : 16
com.xxx.shared.csharpsystem.exception.MyException_FieldSerializer Method name : create LINE_NUMER : 25
com.google.gwt.user.client.rpc.impl.SerializerBase Method name : instantiate LINE_NUMER : 115
com.google.gwt.user.client.rpc.impl.ClientSerializationStreamReader Method name : deserialize LINE_NUMER : 111
com.google.gwt.user.client.rpc.impl.AbstractSerializationStreamReader Method name : readObject LINE_NUMER : 119
com.google.gwt.user.client.rpc.impl.RequestCallbackAdapter Method name : onResponseReceived LINE_NUMER : 216
com.google.gwt.http.client.Request Method name : fireOnResponseReceived LINE_NUMER : 287
com.google.gwt.http.client.RequestBuilder$1 Method name : onReadyStateChange LINE_NUMER : 395
sun.reflect.NativeMethodAccessorImpl Method name : invoke0 LINE_NUMER : -2
sun.reflect.NativeMethodAccessorImpl Method name : invoke LINE_NUMER : 57
sun.reflect.DelegatingMethodAccessorImpl Method name : invoke LINE_NUMER : 43
java.lang.reflect.Method Method name : invoke LINE_NUMER : 601
com.google.gwt.dev.shell.MethodAdaptor Method name : invoke LINE_NUMER : 103
com.google.gwt.dev.shell.MethodDispatch Method name : invoke LINE_NUMER : 71
com.google.gwt.dev.shell.OophmSessionHandler Method name : invoke LINE_NUMER : 172
com.google.gwt.dev.shell.BrowserChannelServer Method name : reactToMessagesWhileWaitingForReturn LINE_NUMER : 337
com.google.gwt.dev.shell.BrowserChannelServer Method name : invokeJavascript LINE_NUMER : 218
com.google.gwt.dev.shell.ModuleSpaceOOPHM Method name : doInvoke LINE_NUMER : 136
com.google.gwt.dev.shell.ModuleSpace Method name : invokeNative LINE_NUMER : 561
com.google.gwt.dev.shell.ModuleSpace Method name : invokeNativeObject LINE_NUMER : 269
com.google.gwt.dev.shell.JavaScriptHost Method name : invokeNativeObject LINE_NUMER : 91
com.google.gwt.core.client.impl.Impl Method name : apply LINE_NUMER : -1
com.google.gwt.core.client.impl.Impl Method name : entry0 LINE_NUMER : 213
sun.reflect.NativeMethodAccessorImpl Method name : invoke0 LINE_NUMER : -2
sun.reflect.NativeMethodAccessorImpl Method name : invoke LINE_NUMER : 57
sun.reflect.DelegatingMethodAccessorImpl Method name : invoke LINE_NUMER : 43
java.lang.reflect.Method Method name : invoke LINE_NUMER : 601
com.google.gwt.dev.shell.MethodAdaptor Method name : invoke LINE_NUMER : 103
com.google.gwt.dev.shell.MethodDispatch Method name : invoke LINE_NUMER : 71
com.google.gwt.dev.shell.OophmSessionHandler Method name : invoke LINE_NUMER : 172
com.google.gwt.dev.shell.BrowserChannelServer Method name : reactToMessages LINE_NUMER : 292
com.google.gwt.dev.shell.BrowserChannelServer Method name : processConnection LINE_NUMER : 546
com.google.gwt.dev.shell.BrowserChannelServer Method name : run LINE_NUMER : 363
java.lang.Thread Method name : run LINE_NUMER : 722
and the code of file MyException.java is :
public class MyException extends Exception implements java.io.Serializable
{
public MyException ()
{
super();
}
public MyException (String Exception, String StackTrace)
{
super(Exception);
if (!StackTrace.equals(""))
{
StackTraceElement[] xStackTraceElementColl = new StackTraceElement[1];
String strDeclaringClass = "";
String strMethodName = StackTrace;
String strFileName = "";
int iLineNo = 0;
xStackTraceElementColl[0] = new StackTraceElement(strDeclaringClass, strMethodName, strFileName, iLineNo);
this.setStackTrace(xStackTraceElementColl);
}
}
}
I don't know why this happen.
thank you for your help.