使用DLL时如何处理“System.NullReferenceException”类型的异常[1]

我练习使用api32写了一个dll，在任意窗口点击鼠标左键可查看该窗口的类名，可顺利通过调试。但程序运行一会就出现‘未处理的“system.nullreferenceexception”类型的异常出现在 system.windows.forms.dll 中。其他信息: 未将对象引用设置到对象的实例。’谁能给分析一下，这是什么原因？
先将源码附上，请大家共同探讨。

//以下为dll类库源码
using system;
using system.runtime.interopservices;
using system.text;

namespace mhook
{
 /// <summary>
 /// class1 的摘要说明。
 /// </summary>
 public class mousehook
 {
  public mousehook()
  {
   //
   // todo: 在此处添加构造函数逻辑
   //
  }

#region         //********api32函数声明**************

  // **********************************************************************

  // win32: setwindowshookex()
  [dllimport("user32.dll",charset=charset.auto,
    callingconvention=callingconvention.stdcall)]
  private static extern intptr setwindowshookex(hooktype code,
   hookproc func,
   intptr hinstance,
   int threadid);

    // win32: unhookwindowshookex()
  [dllimport("user32.dll",charset=charset.auto,
    callingconvention=callingconvention.stdcall)]
  private static extern int unhookwindowshookex(intptr hhook);

  // win32: callnexthookex()
  [dllimport("user32.dll",charset=charset.auto,
    callingconvention=callingconvention.stdcall)]
  private static extern int callnexthookex(intptr hhook,
   int ncode, int wparam, intptr lparam);

     
  // win32: getmodulehandle()
  [dllimport("kernel32.dll",charset=charset.auto,
    callingconvention=callingconvention.stdcall)]
  private static extern intptr getmodulehandle(string lpmodulename);

  
  // win32: getforegroundwindow()
  [dllimport( "user32.dll",charset=charset.auto,
    callingconvention=callingconvention.stdcall)]
  private static extern intptr getforegroundwindow();

  // win32: getwindowthreadprocessid()
  [dllimport("user32.dll",charset=charset.auto)]
  private static extern int getwindowthreadprocessid (
   intptr hwnd,
   int lpdwprocessid
   );

  
  
  // win32: getcurrentthreadid
  [dllimport("kernel32.dll", entrypoint="getcurrentthreadid")]
  public static extern int getcurrentthreadid ();

    
  // win32: attachthreadinput()
  [dllimport("user32.dll",charset=charset.auto)]
  private static extern int attachthreadinput (
   int idattach,
   int idattachto,
   int fattach
   );

   // win32: getfocus()
  [dllimport("user32.dll",charset=charset.auto)]
  private static extern intptr getfocus ();

    // win32: sendmessage()
  [dllimport("user32.dll",charset=charset.auto)]
  private static extern int sendmessage (
   handleref hwnd,
   int wmsg,
   int wparam,
   int lparam
   );

  [dllimport("user32.dll", entrypoint="sendmessage",charset=charset.auto)]
  public static extern int sendmessagestring (
   handleref hwnd,
   int wmsg,
   int wparam,
   stringbuilder lparam
   );
  
  
  // win32: getwindowtext()
  [dllimport("user32.dll",charset=charset.auto)]
  private static extern int getwindowtext (
   intptr hwnd,
   stringbuilder lpstring,
   int nmaxcount
   );

    
  // win32: getclassname()
  [dllimport("user32.dll",charset=charset.auto)]
  private static extern int getclassname (
   intptr hwnd,
   stringbuilder lpclassname,
   int nmaxcount
   );

   
  
#endregion  

  //enum hooktype  
  private enum hooktype : int
  {
   //wh_journalrecord = 0,
   //wh_journalplayback = 1,