Bug 1746: Failure when shutting down Windows

https://winscp.net/tracker/1746
(cherry picked from commit 6bfdce4c9b102875b1986c234d11157c43a415d7)

Source commit: d89bbb239f5384d722c2c3b6cfae1b54a0ff4365

source/forms/CustomScpExplorer.cpp

@@ -305,6 +305,8 @@ __fastcall TCustomScpExplorerForm::~TCustomScpExplorerForm()
 {
   // this has to be one of the very first things to do
   StopUpdateThread();
+  // This is needed when shuting down Windows only. Otherwise it's already set NULL from Execute()
+  TTerminalManager::Instance()->ScpExplorer = NULL;
 
   delete FCustomCommandLocalFileList;
   delete FCustomCommandRemoteFileList;