add include guard to frequency dock access

KDDW doesn't like access to non existing elements. This happens if
hotspot is compiled without QCustomPlot. This patches fixes that by
adding an compile time check.

Fixes: #575

src/resultspage.cpp

@@ -298,12 +298,15 @@ QMenu* ResultsPage::exportMenu() const
 
 QList<QAction*> ResultsPage::windowActions() const
 {
-    auto ret = QList<QAction*> {m_summaryPageDock->toggleAction(),  m_bottomUpDock->toggleAction(),
-                                m_topDownDock->toggleAction(),      m_flameGraphDock->toggleAction(),
-                                m_callerCalleeDock->toggleAction(), m_disassemblyDock->toggleAction(),
-                                m_timeLineDock->toggleAction()};
-    if (m_frequencyDock)
-        ret.append(m_frequencyDock->toggleAction());
+    auto ret = QList<QAction*>
+    {
+        m_summaryPageDock->toggleAction(), m_bottomUpDock->toggleAction(), m_topDownDock->toggleAction(),
+            m_flameGraphDock->toggleAction(), m_callerCalleeDock->toggleAction(), m_disassemblyDock->toggleAction(),
+            m_timeLineDock->toggleAction(),
+#if QCustomPlot_FOUND
+            m_frequencyDock->toggleAction()
+#endif
+    };
     return ret;
 }
 
@@ -334,8 +337,17 @@ void ResultsPage::initDockWidgets(const QVector<CoreDockWidget*>& restored)
 
     Q_ASSERT(restored.contains(summaryPageDock));
 
-    const auto docks = {m_bottomUpDock, m_topDownDock,     m_flameGraphDock, m_callerCalleeDock,
-                        m_timeLineDock, m_disassemblyDock, m_frequencyDock};
+    const auto docks = {
+        m_bottomUpDock,
+        m_topDownDock,
+        m_flameGraphDock,
+        m_callerCalleeDock,
+        m_timeLineDock,
+        m_disassemblyDock,
+#if QCustomPlot_FOUND
+        m_frequencyDock
+#endif
+    };
     for (auto dock : docks) {
         auto dockWidget = toDockWidget(dock);