msi: Create the MSI hidden window on demand.

dlls/msi/dialog.c

@@ -3482,6 +3482,9 @@ msi_dialog *msi_dialog_create( MSIPACKAGE* package,
 
     TRACE("%p %s\n", package, debugstr_w(szDialogName));
 
+    if (!hMsiHiddenWindow)
+        msi_dialog_register_class();
+
     /* allocate the structure for the dialog to use */
     dialog = msi_alloc_zero( sizeof *dialog + sizeof(WCHAR)*strlenW(szDialogName) );
     if( !dialog )

dlls/msi/msi_main.c

@@ -72,7 +72,6 @@ BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpvReserved)
     case DLL_PROCESS_ATTACH:
         msi_hInstance = hinstDLL;
         DisableThreadLibraryCalls(hinstDLL);
-        msi_dialog_register_class();
         break;
     case DLL_PROCESS_DETACH:
         if (msi_typelib) ITypeLib_Release( msi_typelib );