check JSpecify (https://jspecify.dev) - it's the standardised null annotation package for Java. Intellij understands the annotations so you generally get decent null-checking across your codebase.

Even better, apply at the package level via `package-info.java` (unfortunately sub-packages need to be individually marked as well)

  @NullMarked
  package com.foo;

  import org.jspecify.annotations.NullMarked;