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.