Is it possible to add a widget in the foreground without a stack?

I want to create a error dialog on the bottom like on the screenshot at the end of the question.

One solution is to wrap the whole scaffold in a stack and on an error event add this error widget to the stack. I tried that but this is very laborious when I have a lot of widgets.

Is there a way to create a widget in the foreground (e.g like a dialog), without a stack?

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