Important fix to uiscrollarea

I messed up the code and forgot to commit this.
->Check if function is set before calling it here.

modules/corelib/ui/uiscrollarea.lua

@@ -71,7 +71,7 @@ function UIScrollArea:setVerticalScrollBar(scrollbar)
     local virtualOffset = self:getVirtualOffset()
     virtualOffset.y = value
     self:setVirtualOffset(virtualOffset)
-    self:onScrollbarChange(value)  -- Maybe there is a better way to do this?
+    if self.onScrollbarChange then self:onScrollbarChange(value) end
   end
   self:updateScrollBars()
 end
@@ -82,7 +82,7 @@ function UIScrollArea:setHorizontalScrollBar(scrollbar)
     local virtualOffset = self:getVirtualOffset()
     virtualOffset.x = value
     self:setVirtualOffset(virtualOffset)
-    self:onScrollbarChange(value)  -- Maybe there is a better way to do this?
+    if self.onScrollbarChange then self:onScrollbarChange(value) end
   end
   self:updateScrollBars()
 end