Well, you semed to have had a really, really old version of Flutter (and Dart) before.
One way to fix those errors is to read and understand the guide to null safety and modify your project. It will be better after you do it, null safety is an amazing feature.
If you don’t have the time to do that right now, you should be able to just constrain your SDK to an earlier version. Quite frankly I’m surprised that you got null safety out of the box because your project should have had constraints. If you set the minimum SDK version to something smaller than 2.12 you should be running without that feature. But please note that sooner or later you will need to enable it, you cannot stay on an obsolete version. So maybe it’s time to do that now.
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