Fix crash when clientid is invalid

src/client/thing.h

@@ -59,6 +59,8 @@ public:
     virtual bool isAnimatedText() { return false; }
     virtual bool isStaticText() { return false; }
 
+    virtual bool isValid() { return rawGetThingType() != nullptr; }
+
     // type shortcuts
     virtual const ThingTypePtr& getThingType();
     virtual ThingType *rawGetThingType();

src/client/uiitem.cpp

@@ -44,7 +44,7 @@ void UIItem::drawSelf(Fw::DrawPane drawPane)
 
     drawImage(m_rect);
 
-    if(m_itemVisible && m_item) {
+    if(m_itemVisible && m_item && m_item->isValid()) {
         Rect drawRect = getPaddingRect();
         Point dest = drawRect.bottomRight() + Point(1,1);