toString throws NullPointerException

This error occurs when the class under test can throw a NullPointerException when one of its fields is null and equals/hashCode/toString is called. For example, equals could contain this line:

return foo.equals(other.foo);

It will throw a NullPointerException if foo is null. This can be avoided in multiple ways:

In Java 7 and up, you can use Objects.equals, which is null-safe:

return Objects.equals(foo, other.foo);

You can add a null check, like so:

return foo == null ? other.foo == null : foo.equals(other.foo);

If you’re certain the field can never be null (for instance, because the class’s constructor explicitly checks for it), you can add .withNonnullFields("foo") to your call to EqualsVerifier.
If you use @NonNull/@NotNull/@Nullable or similar annotations in your code, then make sure the annotations are available on the classpath when the test is executed.
If this problem occurs for many fields in the same class, you can suppress Warning.NULL_FIELDS in your call to EqualsVerifier.

You can read more about dealing with null in the manual page about null.