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When I try to build my app (which contains one Portable/PCL project and one Android project), I get:
Error Unexpected error - Please file a bug report at http://bugzilla.xamarin.com. Reason: System.IO.FileNotFoundException: Could not load assembly 'UX Prototype Viewer, Version=, Culture=neutral, PublicKeyToken='. Perhaps it doesn't exist in the Mono for Android profile? UXPrototypeViewer.Droid
Visual Studio 2015 particulars:
Microsoft Visual Studio Enterprise 2015
Version 14.0.25123.00 Update 2
Microsoft .NET Framework
Installed Version: Enterprise
Architecture and Modeling Tools 00322-80000-00000-AA873
Microsoft Architecture and Modeling Tools
UML® and Unified Modeling Language™ are trademarks or registered trademarks of the Object Management Group, Inc. in the United States and other countries.
Visual Basic 2015 00322-80000-00000-AA873
Microsoft Visual Basic 2015
Visual C# 2015 00322-80000-00000-AA873
Microsoft Visual C# 2015
Visual C++ 2015 00322-80000-00000-AA873
Microsoft Visual C++ 2015
Windows Phone SDK 8.0 - ENU 00322-80000-00000-AA873
Windows Phone SDK 8.0 - ENU
Application Insights Tools for Visual Studio Package 5.204.60502.1
Application Insights Tools for Visual Studio
ASP.NET and Web Tools 2015.1 (Beta8) 14.1.11106.0
ASP.NET and Web Tools 2015.1 (Beta8)
ASP.NET Web Frameworks and Tools 2012.2 4.1.41102.0
For additional information, visit http://go.microsoft.com/fwlink/?LinkID=309563
ASP.NET Web Frameworks and Tools 2013 5.2.40314.0
For additional information, visit http://www.asp.net/
Common Azure Tools 1.7
Provides common services for use by Azure Mobile Services and Microsoft Azure Tools.
Templates for developing cross-platform applications using the iFactr framework.
InstallShield Limited Edition
InstallShield. For more information visit the Flexera Software website at <http://www.FlexeraSoftware.com>. Copyright © 2015 Flexera Software LLC. All Rights Reserved.
Microsoft Azure Mobile Services Tools 1.4
Microsoft Azure Mobile Services Tools
NuGet Package Manager 3.4.3
NuGet Package Manager in Visual Studio. For more information about NuGet, visit http://docs.nuget.org/.
PreEmptive Analytics Visualizer 1.2
Microsoft Visual Studio extension to visualize aggregated summaries from the PreEmptive Analytics product.
SQL Server Data Tools 14.0.60311.1
Microsoft SQL Server Data Tools
TypeScript tools for Visual Studio
Visual Studio Tools for Universal Windows Apps 14.0.25208.00
The Visual Studio Tools for Universal Windows apps allow you to build a single universal app experience that can reach every device running Windows 10: phone, tablet, PC, and more. It includes the Microsoft Windows 10 Software Development Kit.
Xamarin 188.8.131.52 (0dd817c)
Visual Studio extension to enable development for Xamarin.iOS and Xamarin.Android.
Xamarin.Android 184.108.40.206 (a94a03b)
Visual Studio plugin to enable development for Xamarin.Android.
Xamarin.iOS 220.127.116.11 (3a25bf1)
Visual Studio extension to enable development for Xamarin.iOS.
Make sure that you have set the "UX Prototype Viewer" project to "Build" in the Configuration Manager for the current configuration, and make sure that it builds successfully. The error message means that Xamarin.Android could not find the `.dll` file for "UX Prototype Viewer", and that most often just means that the missing `.dll` file did not build successfully. I will tentatively mark this bug as a duplicate of Bug 40782 based on this answer.
If that explanation does not account for the symptoms in your case, please attach the Diagnostic MSBuild Output  for the failing build onto this bug report and change the bug status to "Reopened".
*** This bug has been marked as a duplicate of bug 40782 ***