Fix possible crash creating an unknown dock widget

if an unknown string was passed to createInnerDockidget,
the widget would not be created but a reference to the null
pointer would still be saved on the avaliable dock widgets,
and thus any code that foreached the list accessing methods
could led to a crash.
Signed-off-by: Tomaz Canabrava <tomaz.canabrava@intel.com>

src/ui/MainWindow.cc

@@ -389,6 +389,7 @@ void MainWindow::_createInnerDockWidget(const QString& widgetName)
         widget = pInfoView;
     } else {
         qWarning() << "Attempt to create unknown Inner Dock Widget" << widgetName;
+	return;
     }
     
     _mapName2DockWidget[widgetName] = widget;