Update MainDlg.h

[shalala66]

The handle has been initialized to "nullptr", because of it defined as one of the expectations on the below in winnt.h (WinNT.h):

// begin_ntoshvp

// // Handle to an Object //

#ifdef STRICT typedef void *HANDLE; #if 0 && (_MSC_VER > 1000) #define DECLARE_HANDLE(name) struct name##; typedef struct name## *name #else #define DECLARE_HANDLE(name) struct name##{int unused;}; typedef struct name## *name #endif #else typedef PVOID HANDLE; #define DECLARE_HANDLE(name) typedef HANDLE name #endif typedef HANDLE *PHANDLE;

// end_ntoshvp

Which also referenced from an official sources:

https://learn.microsoft.com/en-us/windows/win32/winprog/windows-data-types?redirectedfrom=MSDN#:~:text=short%20HALF_PTR%3B%0A%23endif-,HANDLE,-A%20handle%20to

https://learn.microsoft.com/en-us/cpp/extensions/nullptr-cpp-component-extensions?view=msvc-170#:~:text=used%20anywhere%20a-,handle,-%2C%20native%20pointer%2C%20or