gui: Drop ShutdownWindow dependency to BitcoinGUI

src/qt/utilitydialog.cpp

@@ -10,8 +10,6 @@
 
 #include <qt/forms/ui_helpmessagedialog.h>
 
-#include <qt/bitcoingui.h>
-
 #include <clientversion.h>
 #include <init.h>
 #include <util/system.h>
@@ -21,9 +19,10 @@
 
 #include <QCloseEvent>
 #include <QLabel>
+#include <QMainWindow>
 #include <QRegExp>
-#include <QTextTable>
 #include <QTextCursor>
+#include <QTextTable>
 #include <QVBoxLayout>
 
 /** "Help message" or "About" dialog box */
@@ -144,10 +143,9 @@ ShutdownWindow::ShutdownWindow(QWidget *parent, Qt::WindowFlags f):
     setLayout(layout);
 }
 
-QWidget *ShutdownWindow::showShutdownWindow(BitcoinGUI *window)
+QWidget* ShutdownWindow::showShutdownWindow(QMainWindow* window)
 {
-    if (!window)
-        return nullptr;
+    assert(window != nullptr);
 
     // Show a simple window indicating shutdown status
     QWidget *shutdownWindow = new ShutdownWindow();