The lower bound of “sdk: ‘>=2.1.0 <3.0.0′” must be 2.12.0 or higher to enable null safety
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You must update your flutter/dart version like this (in your pubspec.yaml):
In order to enable null safety, the lower bound of the SDK version should be 2.12.0 or higher. The provided requirement, “sdk: ‘>=2.1.0 <3.0.0′”, does not meet this criteria, as the minimum required version is 2.12.0. To enable null safety, you should update the SDK version to at least 2.12.0.
environment: sdk: ">=2.7.0 <3.0.0"
You must update your flutter/dart version like this (in your pubspec.yaml):
environment:
sdk: ">=2.18.0"
Be prepared to make changes on your project/code due null safety
Or seek to find a lower version of that “dropdown_formfield” that can be supported by your actual flutter/dart version.