线程“main”中的异常java.lang.RuntimeException：无法编译的源代码 - 错误的sym类型：javax.swing.JFrame.setContentPaneel

https://puu.sh/hLNH1/74b40dff89.png

I have tried googling this issue for many hours been asking around on Skype I haven't found the solution yet. So I think it's time to ask professional developers what's going on. I have tr