Fix NPE When Invoking Methods on Object Fields

[rcosta358]

Description

This PR fixes a null pointer exception when invoking a method through an object field.

Example

public class Test {

    Obj obj = new Obj();

    public void test() {
        obj.foo(); // crash
    }
}

class Obj {
    void foo() {}
}

Related Issue

None.

Type of change

• Bug fix
• New feature
• Documentation update
• Code refactoring

Checklist

• Added/updated tests under liquidjava-example/src/main/java/testSuite/ (Correct* / Error*)
• mvn test passes locally
• Updated docs/README if behavior or API changed