fixed compile issues on some widgets

app/widget/playbackcontrols/playbackcontrols.h

@@ -37,7 +37,7 @@ class PlaybackControls : public QWidget
 {
   Q_OBJECT
 public:
-  PlaybackControls(QWidget* parent);
+  PlaybackControls(QWidget* parent = nullptr);
 
   /**
    * @brief Set whether the timecodes should be shown or not

app/widget/viewer/viewerglwidget.h

@@ -52,7 +52,7 @@ public:
    *
    * QWidget parent.
    */
-  ViewerGLWidget(QWidget* parent);
+  ViewerGLWidget(QWidget* parent = nullptr);
 
   virtual ~ViewerGLWidget() override;