蓦然回首，那人却在，灯火阑珊处。

This error is a well known symptom indicating a build problem ( usually mixing debug/release ) on Windows.

https://www.qtcentre.org/threads/61483-How-to-fix-error-Must-construct-a-QApplication-before-a-QWidget?p=272136#post272136

亏我查了好久的代码逻辑，但却跳过了就应该第一步需要检查的。。。