Component used: Fragment
Version used: androidx.fragment:fragment:1.2.0
Devices/Android versions reproduced on:

Calling FragmentManager toString() when it is not attached to an Activity crashes with NPE since mHost is null here:

@NonNull
@Override
public String toString() {
StringBuilder sb = new StringBuilder(128);
sb.append("FragmentManager{");
sb.append(Integer.toHexString(System.identityHashCode(this)));
sb.append(" in ");
if (mParent != null) {
Class<?> cls = mParent.getClass();
sb.append(cls.getSimpleName());
sb.append("{");
sb.append(Integer.toHexString(System.identityHashCode(mParent)));
sb.append("}");
} else {
Class<?> cls = mHost.getClass();
sb.append(cls.getSimpleName());
sb.append("{");
sb.append(Integer.toHexString(System.identityHashCode(mHost)));
sb.append("}");
}
sb.append("}}");
return sb.toString();
}

Originally detected with a dirty hack of dump()ing a Fragment to detect if it's in backstack or not, as in https://github.com/sockeqwe/mosby/issues/318#issuecomment-433439830