What causes "Parent view may not be null" in umano/AndroidSlidingUpPanel?

Always pass a non-null ViewGroup (usually 'this' from the custom view, not getParent())

umano/AndroidSlidingUpPanel · error · java.lang.IllegalArgumentException

Parent view may not be null

Error message

Parent view may not be null

What it means

ViewDragHelper's private constructor validates its arguments; creating a helper with a null parent ViewGroup throws IllegalArgumentException. The parent is the view whose drag events the helper monitors, so it is mandatory.

Solutions

Always pass a non-null ViewGroup (usually 'this' from the custom view, not getParent())
Create the helper in onAttachedToWindow or the constructor with a valid reference
Null-check/initialize the view before helper creation

Example fix

// before
mDragHelper = ViewDragHelper.create((ViewGroup) getParent(), callback);
// after
mDragHelper = ViewDragHelper.create(this, callback); // inside the custom ViewGroup

Defensive patterns

Strategy: type-guard

Validate before calling

ViewGroup parent = (parentView != null) ? parentView : this;
mDragHelper = ViewDragHelper.create(parent, sensitivity, callback);

Type guard

ViewGroup safeParent(ViewGroup v) { return v != null ? v : (ViewGroup) requireNotNull(getParent(), "not attached yet"); }

Prevention

Pass 'this' from the custom ViewGroup, not getParent()
Create the helper in onAttachedToWindow if parent needed
Never create helper before view inflation

When it happens

Trigger: Calling ViewDragHelper.create(null, cb) or any create(...) overload with a null forParent — typically when a view's getParent() is null (view not attached) or a field is not yet initialized.

Common situations: Constructing the helper in a custom View's field initializer before the parent exists, or passing a detached view's null parent.

Related errors

AI-assisted analysis of umano/AndroidSlidingUpPanel@45a460435b (2026-09-11). Data as JSON: /api/errors/6421b468ba8d2d52. Report an issue: GitHub.

Appendix: source

Thrown at library/src/main/java/com/sothree/slidinguppanel/ViewDragHelper.java:406

    public static ViewDragHelper create(ViewGroup forParent, float sensitivity, Interpolator interpolator, Callback cb) {
        final ViewDragHelper helper = create(forParent, interpolator, cb);
        helper.mTouchSlop = (int) (helper.mTouchSlop * (1 / sensitivity));
        return helper;
    }

    /**
     * Apps should use ViewDragHelper.create() to get a new instance.
     * This will allow VDH to use internal compatibility implementations for different
     * platform versions.
     * If the interpolator is null, the default interpolator will be used.
     *
     * @param context Context to initialize config-dependent params from
     * @param forParent Parent view to monitor
     * @param interpolator interpolator for scroller
     */
    private ViewDragHelper(Context context, ViewGroup forParent, Interpolator interpolator, Callback cb) {
        if (forParent == null) {
            throw new IllegalArgumentException("Parent view may not be null");
        }
        if (cb == null) {
            throw new IllegalArgumentException("Callback may not be null");
        }

        mParentView = forParent;
        mCallback = cb;

        final ViewConfiguration vc = ViewConfiguration.get(context);
        final float density = context.getResources().getDisplayMetrics().density;
        mEdgeSize = (int) (EDGE_SIZE * density + 0.5f);

        mTouchSlop = vc.getScaledTouchSlop();
        mMaxVelocity = vc.getScaledMaximumFlingVelocity();
        mMinVelocity = vc.getScaledMinimumFlingVelocity();
        mScroller = ScrollerCompat.create(context, interpolator != null ? interpolator : sInterpolator);
    }

View on GitHub (pinned to 45a460435b)