問題：

Installation failed with message null. It is possible that this issue is resolved by uninstalling an existing version of the apk if it is present, and then re-installing.

WARNING: Uninstalling will remove the application data!

Do you want to uninstall the existing application?

解決（兩個方法）：

 1.小米手機的問題，開啟設定，開啟開發者選項，關閉MIUI優化

 2.Android studio 的問題 File→Settings→Build，Execution→Debugger→Instant Run,之後將勾去掉。

通過上面兩步，我的問題解決了，不過之後重新開啟MIUI優化和打上勾，也沒有出現問題（這是為啥沒搞清楚）。