fix tap on word title will crash koreader

frontend/ui/uimanager.lua

@@ -249,7 +249,9 @@ function UIManager:setDirty(widget, refresh_type)
     if not refresh_type then
         refresh_type = "auto"
     end
-    self._dirty[widget] = refresh_type
+    if widget then
+        self._dirty[widget] = refresh_type
+    end
 end
 
 function UIManager:insertZMQ(zeromq)

frontend/ui/widget/button.lua

@@ -147,7 +147,7 @@ function Button:showHide(show)
 end
 
 function Button:onTapSelect()
-    if self.enabled then
+    if self.enabled and self.callback then
         self[1].invert = true
         UIManager:setDirty(self.show_parent, "partial")
         UIManager:scheduleIn(0.1, function()