Fix #1834: segfault on addMaximizeButton

... for image/camera displays if no window manager is instantiated

Author: rhaschke

src/rviz/default_plugin/image_display.cpp

@@ -131,7 +131,8 @@ void ImageDisplay::onInitialize()
   render_panel_->initialize(img_scene_manager_, context_);
 
   setAssociatedWidget(render_panel_);
-  getAssociatedWidgetPanel()->addMaximizeButton();
+  if (auto* dock = getAssociatedWidgetPanel())
+    dock->addMaximizeButton();
 
   render_panel_->setAutoRender(false);
   render_panel_->setOverlaysEnabled(false);