Verifier crashes when an enum (or class) constant is compared in an `if`

[CatarinaGamboa]

Description

Comparing a value with a constant accessed through a type, directly in an if condition, crashes verification. Seen with a user enum (format == Format.JPG) and with JDK constants (mode != ImageWriteParam.MODE_COPY_FROM_METADATA). Assigning the comparison to a boolean local first works.

Minimal reproducer (no refinements needed)

public class Repro {
    enum Format { JPG, PNG }

    static String extension(Format format) {
        if (format == Format.JPG) {
            return "jpg";
        }
        return "png";
    }

    public static void main(String[] args) {
        System.out.println(extension(Format.PNG));
    }
}

Expected

Correct! Passed Verification.

Actual

Error while checking CtIfImpl
  on if (format == Format.JPG) {
  with Cannot invoke "liquidjava.rj_language.Predicate.substituteVariable(String, String)" because "elemRef" is null

Reproduced on main at 8816186 (liquidjava-verifier 0.0.35), and on 0.0.33.

Context

Found while turning real open-source Java code into study examples for the error-message study: real code hits this constantly, so the examples have to be rewritten around it or cannot be verified. Affects 2 of our examples.