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How do you debug a crash from an invalid UObject reference?

This tracker crashes a few seconds after an enemy dies: UpdateScore dereferences a nullptr or garbage address. The check if (LastKilledEnemy) does not help — the pointer is non-null yet the memory is freed.

// AScoreTracker.h
UCLASS()
class AScoreTracker : public AActor
{
    GENERATED_BODY()
public:
    void TrackEnemy(AEnemy* Enemy);
    void UpdateScore();

private:
    AEnemy* LastKilledEnemy = nullptr;
};

// AScoreTracker.cpp
void AScoreTracker::TrackEnemy(AEnemy* Enemy)
{
    LastKilledEnemy = Enemy;
}

void AScoreTracker::UpdateScore()
{
    int32 Bonus = LastKilledEnemy->GetScoreValue();  // crash
}

Find and fix the bug.

Read the callstack to find the dereferenced pointer and check whether it is a UPROPERTY(). A non-reflected pointer dangling after GC is the usual cause; fix it by adding UPROPERTY() or using TWeakObjectPtr with an IsValid guard.

✗Removing UPROPERTY() to 'stop the GC nulling' — that causes the dangle
✗Assuming the crash is build-config specific rather than a real lifetime bug
✗Forcing CollectGarbage() instead of fixing the missing reference

→How would gc.PendingKillEnabled or stomp allocator help isolate the crash?
→Why does adding UPROPERTY() change a crash into a safe null dereference you can guard?

Contents

Symptom

The game crashes at random moments, often a few seconds after an enemy is destroyed. The callstack points at a nullptr or garbage-address dereference inside a method.

Buggy code

// AScoreTracker.h
UCLASS()
class AScoreTracker : public AActor
{
    GENERATED_BODY()
public:
    void TrackEnemy(AEnemy* Enemy);
    void UpdateScore();

private:
    // BUG: raw pointer with no UPROPERTY()
    AEnemy* LastKilledEnemy = nullptr;
};

// AScoreTracker.cpp
void AScoreTracker::TrackEnemy(AEnemy* Enemy)
{
    LastKilledEnemy = Enemy;            // stored a raw pointer
}

void AScoreTracker::UpdateScore()
{
    // A few frames later the enemy is destroyed and collected.
    // LastKilledEnemy now dangles — the GC never nulled it.
    int32 Bonus = LastKilledEnemy->GetScoreValue();  // CRASH
}

Diagnosis

Because LastKilledEnemy is not marked UPROPERTY(), the garbage collector cannot see this reference. When the enemy becomes unreachable and is collected, the GC does not null the pointer — it dangles. An if (LastKilledEnemy) check is useless: the pointer is non-null but the memory is already freed. The crash is delayed, so the callstack points at UpdateScore, not at the real cause.

Fix

private:
    // Option A: strong reference — UPROPERTY() keeps the enemy alive
    //           and nulls the pointer when the enemy is destroyed.
    UPROPERTY()
    AEnemy* LastKilledEnemy = nullptr;

    // Option B: weak reference — does not keep the enemy alive,
    //           but reports invalidity safely.
    TWeakObjectPtr<AEnemy> LastKilledEnemyWeak;

void AScoreTracker::UpdateScore()
{
    // With option A: the check now actually works.
    if (IsValid(LastKilledEnemy))
    {
        int32 Bonus = LastKilledEnemy->GetScoreValue();
    }
    // With option B:
    if (AEnemy* Enemy = LastKilledEnemyWeak.Get())
    {
        int32 Bonus = Enemy->GetScoreValue();
    }
}

The choice depends on intent: if the tracker should logically extend the enemy's lifetime, use option A; if it merely observes and the enemy may vanish at any time, use option B.