Fixing ComboBox handlers and accessibility object initialization

[M-Lipin]

Adding IsHandleCreated checks to prevent accessing AccessibleObject properties and calls to UIAutomation which may lead to application crash/hung.

Fixes #2432

Proposed changes

• Adding IsHandleCreated checks to prevent accessing AccessibleObject properties for not fully initialized control.
• Fixed the reference to child list window to prevent NullReferenceException on child control recreating.

Customer Impact

• Customers will not experience application crash/hung in some cases when ComboBox control is being reinitialized while application/form lifecycle.

Regression?

• Yes

Risk

• Minimal

Screenshots

Before

Exception and application hung.

NullReferenceException and application crash:

System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Windows.Forms.ComboBox.WmParentNotify(Message& m)
   at System.Windows.Forms.ComboBox.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

After

No exception.

Test methodology

• Manual testing.
• Automation tests.

Test environment(s)

.NET Core SDK (reflecting any global.json):
Version: 5.0.100-alpha1-015763
Commit: 0d0c902b77

Runtime Environment:
OS Name: Windows
OS Version: 10.0.18363
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\5.0.100-alpha1-015763\

Host (useful for support):
Version: 5.0.0-alpha.1.19564.1
Commit: c77948d92a

.NET Core SDKs installed:
3.1.100-preview3-014645 [C:\Program Files\dotnet\sdk]
5.0.100-alpha1-015763 [C:\Program Files\dotnet\sdk]

Microsoft Reviewers: Open in CodeFlow