Spring [question] for beans components #getPropertyHoldingValue

Hey, recently I tried to debug the source code of beans components. branch： remotes/origin/5.1.x file: https://github.com/spring-projects/spring-framework/blob/master/spring-beans/src/main/java/org/springframework/beans/AbstractNestablePropertyAccessor.java line: 391 accessor.setAutoGrowNestedPaths(true);

When I execute here, my keys are non-null, but the getPropertyValue method sets DefaultValue when keys are non-null, so getPropertyValue should return to a non-null collection.

private Object getPropertyHoldingValue(PropertyTokenHolder tokens) {
        // Apply indexes and map keys: fetch value for all keys but the last one.
        Assert.state(tokens.keys != null, "No token keys");
        PropertyTokenHolder getterTokens = new PropertyTokenHolder(tokens.actualName);
        getterTokens.canonicalName = tokens.canonicalName;
        getterTokens.keys = new String[tokens.keys.length - 1];
        System.arraycopy(tokens.keys, 0, getterTokens.keys, 0, tokens.keys.length - 1);
        Object propValue;
        try {
            // call getPropertyValue , keys must be not null
            propValue = getPropertyValue(getterTokens);
        }
        catch (NotReadablePropertyException ex) {
            throw new NotWritablePropertyException(getRootClass(), this.nestedPath + tokens.canonicalName,
                    "Cannot access indexed value in property referenced " +
                    "in indexed property path '" + tokens.canonicalName + "'", ex);
        }

        // why check that propValue is null after calling getPropertyValue？
        if (propValue == null) {
            // null map value case
            if (isAutoGrowNestedPaths()) {
                int lastKeyIndex = tokens.canonicalName.lastIndexOf('[');
                getterTokens.canonicalName = tokens.canonicalName.substring(0, lastKeyIndex);
                propValue = setDefaultValue(getterTokens);
            }
            else {
                throw new NullValueInNestedPathException(getRootClass(), this.nestedPath + tokens.canonicalName,
                        "Cannot access indexed value in property referenced " +
                        "in indexed property path '" + tokens.canonicalName + "': returned null");
            }
        }
        return propValue;
    }

protected Object getPropertyValue(PropertyTokenHolder tokens) throws BeansException {
        String propertyName = tokens.canonicalName;
        String actualName = tokens.actualName;
        PropertyHandler ph = getLocalPropertyHandler(actualName);
        if (ph == null || !ph.isReadable()) {
            throw new NotReadablePropertyException(getRootClass(), this.nestedPath + propertyName);
        }
        try {
            Object value = ph.getValue();

            // key are not-null
            if (tokens.keys != null) {
                if (value == null) {
                    // the AutoGrowNestedPaths enabled
                    if (isAutoGrowNestedPaths()) {
                        // this value not null
                        value = setDefaultValue(new PropertyTokenHolder(tokens.actualName));
                    }
                    else {
                        throw new NullValueInNestedPathException(getRootClass(), this.nestedPath + propertyName,
                                "Cannot access indexed value of property referenced in indexed " +
                                        "property path '" + propertyName + "': returned null");
                    }
                }

The question is why you check that propValue is null after calling getPropertyValue. Isn't this done in getPropertyValue?