Help: Unregister URL handler for qthelp before deleting it

Qt 6 warns if that is not done.

Change-Id: If0b9f36e35544d1a1d8a45b8c06005cef3940ebb
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/help/helpmanager.cpp

@@ -34,6 +34,7 @@
 
 #include <QDateTime>
 #include <QDebug>
+#include <QDesktopServices>
 #include <QDir>
 #include <QFileInfo>
 #include <QStringList>
@@ -109,10 +110,12 @@ HelpManager::HelpManager(QObject *parent) :
     QTC_CHECK(!m_instance);
     m_instance = this;
     d = new HelpManagerPrivate;
+    QDesktopServices::setUrlHandler("qthelp", this, "showHelpUrl");
 }
 
 HelpManager::~HelpManager()
 {
+    QDesktopServices::unsetUrlHandler("qthelp");
     delete d;
     m_instance = nullptr;
 }

src/plugins/help/helpplugin.cpp

@@ -286,7 +286,6 @@ HelpPluginPrivate::HelpPluginPrivate()
     connect(&helpIndexFilter, &HelpIndexFilter::linksActivated,
             this, &HelpPluginPrivate::showLinksInCurrentViewer);
 
-    QDesktopServices::setUrlHandler("qthelp", HelpManager::instance(), "showHelpUrl");
     connect(ModeManager::instance(), &ModeManager::currentModeChanged,
             this, &HelpPluginPrivate::modeChanged);