vshost.exe error when closing my win app, How to deal with it ?

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vshost.exe

I have MDI and Child Forms. My MDI_FormClosing event contains the following code:

clsMain.CN.Close(); //"CN" is my SqlConnection object
clsMain.CN.Dispose();
Environment.Exit(-1);

When I closed the MDI form with no opened child forms, it was working well.
When I try to close MDI and there is a child form is opened, the following exception is thrown.

http://www.mediafire.com/conv/e039f1817d1719f47d55f3a1270ebad95741c775c40d4c36c500422dab7c09d86g.jpg

Problem signature:
  Problem Event Name:   APPCRASH
  Application Name: General Accounts.vshost.exe
  Application Version:  10.0.30319.1
  Application Timestamp:    4ba2084b
  Fault Module Name:    ntdll.dll
  Fault Module Version: 6.1.7600.16385
  Fault Module Timestamp:   4a5bdadb
  Exception Code:   c0000005
  Exception Offset: 00024b7d
  OS Version:   6.1.7600.2.0.0.256.1
  Locale ID:    3073
  Additional Information 1: e8ad
  Additional Information 2: e8adce1c2b9e7be834b4063ac3c53863
  Additional Information 3: e8ad
  Additional Information 4: e8adce1c2b9e7be834b4063ac3c53863

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I tried to go to Project Properties, Debug tab and remove the check from "Enable the visual studio hosting process" .. it is still not working.

How can I avoid this message or this error ??