From fa1f5bc8f9f5e3b91b6a97801910d79e6af41cc5 Mon Sep 17 00:00:00 2001
From: Dan Walmsley <dan@walms.co.uk>
Date: Thu, 13 Jul 2017 15:11:48 +0100
Subject: [PATCH] add fix for null reference exception when arranging.

added comments explaining the conditions that the bug occurs in.
---
 src/Avalonia.Layout/LayoutManager.cs | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/Avalonia.Layout/LayoutManager.cs b/src/Avalonia.Layout/LayoutManager.cs
index 965ab3eee6..1ce55e2afa 100644
--- a/src/Avalonia.Layout/LayoutManager.cs
+++ b/src/Avalonia.Layout/LayoutManager.cs
@@ -190,8 +190,10 @@ namespace Avalonia.Layout
                 {
                     root.Arrange(new Rect(control.DesiredSize));
                 }
-                else
+                else if(control.PreviousArrange != null)
                 {
+                    // Has been observed that PreviousArrange sometimes is null, probably a bug somewhere else.
+                    // Condition observed: control.VisualParent is Scrollbar, control is Border.
                     control.Arrange(control.PreviousArrange.Value);
                 }
             }