Fix datadir handling in bitcoin-qt

This prevents premature tries to access or create the default datadir.
This is useful when the -datadir option is specified and the default
datadir is unreachable.

src/qt/bitcoin.cpp

@@ -495,10 +495,9 @@ int GuiMain(int argc, char* argv[])
     if (!Intro::pickDataDirectory(*node))
         return EXIT_SUCCESS;
 
-    /// 6. Determine availability of data and blocks directory and parse bitcoin.conf
+    /// 6. Determine availability of data directory and parse bitcoin.conf
     /// - Do not call GetDataDir(true) before this step finishes
-    if (!fs::is_directory(GetDataDir(false)))
-    {
+    if (!CheckDataDirOption()) {
         node->initError(strprintf("Specified data directory \"%s\" does not exist.\n", gArgs.GetArg("-datadir", "")));
         QMessageBox::critical(nullptr, PACKAGE_NAME,
             QObject::tr("Error: Specified data directory \"%1\" does not exist.").arg(QString::fromStdString(gArgs.GetArg("-datadir", ""))));