Validation on the input before connect request is triggered...

On the launch page (`devtools_app/lib/src/flutter/connect_screen.dart`), there should be some validation on the input URL, if I for instance enter `random text inserted into connect box` in the `Connect` box where a URL goes, an attempt is made and I see the popup:

`Unable to connect to 'random text insterted into connect box'`

从 devtools_app/lib/src/flutter/connect_screen.dart 开始，在 Connect 框中输入非 URL 文本以重现问题。当无效输入在发起连接尝试之前得到验证，并且用户收到适当的反馈而不是连接弹窗时，即表示完成。

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