title: Xamarin Android axml Designer, System.ArgumentNullException: Value cannot be null
body: <p>Note: This occurs with the most recent version of Visual Studio Mac and the latest Xamarin tools installed.</p>
  
  <p>I started to work on an older Xamarin Android project that I've been maintaining over the past few years. It's been a long time since I've made any UI changes to the application and there was a minor adjustment I needed to make in order to add some additional information to the screen. I opened up the axml layout of screen I needed to modify. When doing so, the designer view displayed the following error:</p>
  
  <blockquote>
    <p>System.ArgumentNullException: Value cannot be null.
    Parameter name: path1
      at System.IO.Path.Combine (System.String path1, System.String path2, System.String path3) [0x0002a] in /Users/builder/data/lanes/4992/mono-mac-sdk/external/bockbuild/builds/mono-x64/mcs/class/corlib/System.IO/Path.cs:849 
      at Xamarin.AndroidDesigner.DesignerProject+&lt;>c.b__118_0 (System.String l) [0x00000] in /Users/vsts/agent/2.126.0/work/1/s/Xamarin.Designer.Android/Xamarin.AndroidDesigner/Xamarin.AndroidDesigner/DesignerProject.cs:532 
      at System.Linq.Enumerable+SelectListIterator<code>2[TSource,TResult].MoveNext () [0x00036] in /Users/builder/data/lanes/4992/mono-mac-sdk/external/bockbuild/builds/mono-x64/external/corefx/src/System.Linq/src/System/Linq/Select.cs:365 
      at System.Linq.Enumerable+WhereEnumerableIterator</code>1[TSource].MoveNext () [0x0004e] in /Users/builder/data/lanes/4992/mono-mac-sdk/external/bockbuild/builds/mono-x64/external/corefx/src/System.Linq/src/System/Linq/Where.cs:146 
      at System.Linq.Enumerable+SelectManySingleSelectorIterator<code>2[TSource,TResult].ToList () [0x0002d] in /Users/builder/data/lanes/4992/mono-mac-sdk/external/bockbuild/builds/mono-x64/external/corefx/src/System.Linq/src/System/Linq/SelectMany.cs:257 
      at System.Linq.Enumerable.ToList[TSource] (System.Collections.Generic.IEnumerable</code>1[T] source) [0x00015] in /Users/builder/data/lanes/4992/mono-mac-sdk/external/bockbuild/builds/mono-x64/external/corefx/src/System.Linq/src/System/Linq/ToCollection.cs:30 
      at Xamarin.AndroidDesigner.DesignerProject.GetLibraryProjectThemes () [0x00037] in /Users/vsts/agent/2.126.0/work/1/s/Xamarin.Designer.Android/Xamarin.AndroidDesigner/Xamarin.AndroidDesigner/DesignerProject.cs:531 
      at Xamarin.AndroidDesigner.DesignerProject.GetIsProjectTheme (System.String themeName) [0x0000e] in /Users/vsts/agent/2.126.0/work/1/s/Xamarin.Designer.Android/Xamarin.AndroidDesigner/Xamarin.AndroidDesigner/DesignerProject.cs:544 
      at Xamarin.AndroidDesigner.DesignerSessionParams.GetData (Xamarin.AndroidDesigner.AndroidDesignerSession session, Xamarin.AndroidDesigner.AndroidRenderSession renderer) [0x00154] in /Users/vsts/agent/2.126.0/work/1/s/Xamarin.Designer.Android/Xamarin.AndroidDesigner/Xamarin.AndroidDesigner/DesignerSessionParams.cs:209 
      at Xamarin.AndroidDesigner.AndroidRenderSession+d__115.MoveNext () [0x00224] in /Users/vsts/agent/2.126.0/work/1/s/Xamarin.Designer.Android/Xamarin.AndroidDesigner/Xamarin.AndroidDesigner/AndroidRenderSession.cs:783 </p>
  </blockquote>
  
  <p>I initially thought there must be some kind of Syntax issue with the newer Xamarin tools but when I tried creating a brand new Android Layout, I received the same error for the designer. The Xamarin Android new layout looks like this:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
      android:orientation="vertical"
      android:layout_width="match_parent"
      android:layout_height="match_parent" /&gt;
  </code></pre>
  
  <p>Because of this, I'm thinking that there has to be some kind of setup option that I'm missing due to this being an older project. Perhaps something to do with the incorrect designer tool path being assigned somewhere? Has anyone come across this error or have some ideas on what to check?</p>
  
  <p><strong>Edit:</strong>
  Additional Info
  Android SDK Location:</p>
  
  <blockquote>
    <p>/Users/mrnope/Library/Developer/Xamarin/android-sdk-macosx</p>
  </blockquote>
  
  <p>Android NDK Location:</p>
  
  <blockquote>
    <p>/Users/mrnope/Library/Developer/Xamarin/android-ndk/android-ndk-r14b</p>
  </blockquote>
  
  <p>Java SDK (JDK) Location:</p>
  
  <blockquote>
    <p>/usr</p>
  </blockquote>
  
  <p><a href="https://i.stack.imgur.com/mEbQz.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/mEbQz.png" alt="Files found"></a>
  Android SDK Tools:
  <a href="https://i.stack.imgur.com/Z730s.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Z730s.png" alt="Android SDK Tools"></a>
  SDK Platforms:
  <a href="https://i.stack.imgur.com/CFJwm.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/CFJwm.png" alt="SDK Platforms"></a></p>