CFI crashes on generic conditional expression in nested generic class

[aosen-xiong]

Minimal Reproducer

SmallRepro.java:

import java.util.ArrayList;
import java.util.List;

class SmallRepro<ID, T> {
    static class Box<ID, T> {}

    static class Worker<ID, T> {
        List<Box<ID, T>> f(List<Box<ID, T>> xs) {
            return xs == null ? new ArrayList<>() : xs;
        }
    }
}

Command

./gradlew dist --console=plain --quiet
./scripts/inference --hacks=true --mode INFER \
  --checker nninf.NninfChecker \
  --solver checkers.inference.solver.MaxSat2TypeSolver \
  SmallRepro.java

Error

Error in AnnotatedTypeMirror.fromExpression(InferenceAnnotatedTypeFactory, condExpr#num0):
Cannot invoke "com.sun.source.util.TreePath.getParentPath()"
because the return value of
"checkers.inference.VariableAnnotator.expensiveBackupGetPath(...)" is null

Expected
Inference should not crash on this valid Java program. If CFI cannot recover a TreePath, it should use a fallback location or report a controlled diagnostic.