ListView System.NullReferenceException

[kericbowman]

Description

This only occurs on Android

I have a listview on a page in my app that is producing a System.NullReferenceException. This happens when its page is not at the top of the navigation stack.

Stack trace:

[MonoDroid] UNHANDLED EXCEPTION:
[MonoDroid] System.NullReferenceException: Object reference not set to an instance of an object.
[MonoDroid] at Xamarin.Forms.Platform.Android.ListViewRenderer+Container.OnMeasure (System.Int32 widthMeasureSpec, System.Int32 heightMeasureSpec) [0x00032] in D:\a\1\s\Xamarin.Forms.Platform.Android\Renderers\ListViewRenderer.cs:513
[MonoDroid] at Android.Views.View.n_OnMeasure_II (System.IntPtr jnienv, System.IntPtr native__this, System.Int32 widthMeasureSpec, System.Int32 heightMeasureSpec) [0x00009] in <3faca578033c421aa590cc3addd3b6d8>:0
[MonoDroid] at (wrapper dynamic-method) System.Object.29(intptr,intptr,int,int)

Steps to Reproduce

Narrowing this down to a repro; just wanted to get it up there.

Expected Behavior

I did not expect a null reference exception

Actual Behavior

a null reference exception

Basic Information

• Version with issue:
• Last known good version:
• IDE:
• Platform Target Frameworks:
  • Android: not sure how to answer this? I support multiple platforms and it seems to happen on all of them

Reproduction Link

Working on it

[kingces95]

Please let us know when you have a zipped reproduction. Thanks.

[kericbowman]

I'm having a hard time reproducing this; is there a way I could debug the xamarin.forms source code? Is there a verbose debugging option I can turn on to get more info?

[kericbowman]

Haven't been able to reproduce so I'm looking at the source. this is the function that crashes:

protected override void OnMeasure(int widthMeasureSpec, int heightMeasureSpec)
{
if (_child == null)
{
	SetMeasuredDimension(0, 0);
	return;
}

VisualElement element = _child.Element;

Context ctx = Context;

var width = (int)ctx.FromPixels(MeasureSpecFactory.GetSize(widthMeasureSpec));

SizeRequest request = _child.Element.Measure(width, double.PositiveInfinity, MeasureFlags.IncludeMargins);
Xamarin.Forms.Layout.LayoutChildIntoBoundingRegion(_child.Element, new Rectangle(0, 0, width, request.Request.Height));

int widthSpec = MeasureSpecFactory.MakeMeasureSpec((int)ctx.ToPixels(width), MeasureSpecMode.Exactly);
int heightSpec = MeasureSpecFactory.MakeMeasureSpec((int)ctx.ToPixels(request.Request.Height), MeasureSpecMode.Exactly);

_child.View.Measure(widthMeasureSpec, heightMeasureSpec);
SetMeasuredDimension(widthSpec, heightSpec);
}

specifically, this line throws the nullreference exception:

SizeRequest request = _child.Element.Measure(width, double.PositiveInfinity, MeasureFlags.IncludeMargins);

Since _child has a null check above, and everything else except "Element" is non-nullable on that line isn't it safe to say that "Element" is null?

I'll make the edit and start the pull request. In the mean time; any ideas why "element" could be null and how I could get a work around going on my end?

[kingces95]

The project is open source and has instructions per platform for how to build it. There is an unofficial bat file .create-nuget that you could use to build the nuget or just create ur own unit test. If u use the bat file, set nuget_dir= path to nuget.exe.

[kericbowman]

I tried following the build tutorial but it's for windows and I'm on a mac. Is there a tutorial for building the nuget package with visual studio for mac?

[kericbowman]

upgraded to the latest 4.0 release and the bug seems to have gone away.

[kericbowman]

And it's back, I'll keep working on a repro

[samhouts]

Thanks, we'll keep watching for it. :)

[kericbowman]

Still can't reproduce but I am narrowing it down.

This happens if the OnMeasure method is called on a listview on a page that has been removed from the navigation stack (not popped, removed). I'm not sure why that method is called on a page that is no longer on the navigation stack. I even set the page to null after popping it and it the onMeasure method is still called.

[kericbowman]

Changed from Navigation.remove to popping the page and the same problem persists.