Using futures in user code causes Fuzzer to generate tests which fail during execution

[tyuldashev]

Description
When code contains usage of CompletableFuture Fuzzer may generate tests which throw unexpected exception during execution.

To Reproduce

1. Install UnitTestBot plugin built from main in IntelliJ IDEA
2. Open UTBot Java project
3. Set Test method generation: Fuzzer = 100%
4. Generate tests for examples.threads.FutureExamples#changingCollectionInFutureWithoutGet
5. Run generated tests

Expected behavior
Tests without exception pass

Actual behavior
Half of generated tests fail.

Screenshots, logs
Code under test

    public int changingCollectionInFutureWithoutGet() {
        List<Integer> values = new ArrayList<>();

        final CompletableFuture<Void> future = CompletableFuture.runAsync(() -> values.add(42));

        return values.get(0);
    }

One of generated tests:

    @Test
    @DisplayName("changingCollectionInFutureWithoutGet:  -> return 42")
    public void testChangingCollectionInFutureWithoutGetReturns42() {
        FutureExamples futureExamples = new FutureExamples();

        int actual = futureExamples.changingCollectionInFutureWithoutGet();

        assertEquals(42, actual);
    }

Exception during execution:

java.lang.IndexOutOfBoundsException: Index 0 out of bounds for length 0

	at java.base/jdk.internal.util.Preconditions.outOfBounds(Preconditions.java:64)
	at java.base/jdk.internal.util.Preconditions.outOfBoundsCheckIndex(Preconditions.java:70)
	at java.base/jdk.internal.util.Preconditions.checkIndex(Preconditions.java:248)
	at java.base/java.util.Objects.checkIndex(Objects.java:372)
	at java.base/java.util.ArrayList.get(ArrayList.java:459)
	at examples.threads.FutureExamples.changingCollectionInFutureWithoutGet(FutureExamples.java:41)
	at examples.threads.FutureExamplesTest.testChangingCollectionInFutureWithoutGetReturns42(FutureExamplesTest.java:23)

Additional context
Symbolic engine doesn't generate tests which should pass.