WorkflowRenderer: Don't attempt to use a null render widget

fix #15

src/Renderer/WorkflowRenderer.cpp

@@ -88,7 +88,8 @@ WorkflowRenderer::setupRenderer( quint32 width, quint32 height, double fps, cons
     m_sourceRenderer = m_source->createRenderer( m_eventWatcher );
     m_sourceRenderer->setName( "WorkflowRenderer" );
     m_sourceRenderer->enableMemoryInput( m_esHandler, getLockCallback(), getUnlockCallback() );
-    m_sourceRenderer->setOutputWidget( (void *) static_cast< RenderWidget* >( m_renderWidget )->id() );
+    if ( m_renderWidget != nullptr )
+        m_sourceRenderer->setOutputWidget( (void *) static_cast< RenderWidget* >( m_renderWidget )->id() );
 }
 
 int