Null reference when active configuration is changed

[mrward]

Null reference exception is logged when the active configuration is changed.

 An unhandled exception has occured. Terminating Xamarin Studio? False
System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.NullReferenceException: Object reference not set to an instance of an object
  at OmniSharp.Dnx.DnxProjectSystem.ChangeConfiguration (System.String config) <0x12dc98610 + 0x00340> in <filename unknown>:0 
  at MonoDevelop.Dnx.DnxProjectService.ActiveConfigurationChanged (System.Object sender, System.EventArgs e) <0x1269e3800 + 0x00072> in <filename unknown>:0 
  at (wrapper delegate-invoke) <Module>:invoke_void_object_EventArgs (object,System.EventArgs)
  at MonoDevelop.Ide.RootWorkspace.OnActiveConfigurationChanged () [0x0000b] in /Users/builder/data/lanes/3422/e879ce52/source/monodevelop/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide/RootWorkspace.cs:108 
  at MonoDevelop.Ide.RootWorkspace.set_ActiveConfigurationId (System.String value) [0x00018] in /Users/builder/data/lanes/3422/e879ce52/source/monodevelop/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide/RootWorkspace.cs:100 
  at MonoDevelop.Components.MainToolbar.MainToolbarController.SelectActiveConfiguration () [0x000a1] in /Users/builder/data/lanes/3422/e879ce52/source/monodevelop/main/src/core/MonoDevelop.Ide/MonoDevelop.Components.MainToolbar/MainToolbarController.cs:323 
  at MonoDevelop.Components.MainToolbar.MainToolbarController.UpdateCombos () [0x000b6] in /Users/builder/data/lanes/3422/e879ce52/source/monodevelop/main/src/core/MonoDevelop.Ide/MonoDevelop.Components.MainToolbar/MainToolbarController.cs:162 
  at MonoDevelop.Components.MainToolbar.MainToolbarController.HandleCurrentSelectedSolutionChanged (System.Object sender, MonoDevelop.Projects.SolutionEventArgs e) [0x00084] in /Users/builder/data/lanes/3422/e879ce52/source/monodevelop/main/src/core/MonoDevelop.Ide/MonoDevelop.Components.MainToolbar/MainToolbarController.cs:450 
  at MonoDevelop.Ide.ProjectOperations.OnCurrentSelectedSolutionChanged (MonoDevelop.Projects.SolutionEventArgs e) [0x0000b] in /Users/builder/data/lanes/3422/e879ce52/source/monodevelop/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide/ProjectOperations.cs:2160 
  at MonoDevelop.Ide.ProjectOperations.set_CurrentSelectedWorkspaceItem (MonoDevelop.Projects.WorkspaceItem value) [0x00030] in /Users/builder/data/lanes/3422/e879ce52/source/monodevelop/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide/ProjectOperations.cs:127 
  at MonoDevelop.Ide.Gui.Pads.ProjectPad.ProjectSolutionPad.OnSelectionChanged (System.Object sender, System.EventArgs args) [0x00031] in /Users/builder/data/lanes/3422/e879ce52/source/monodevelop/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide.Gui.Pads.ProjectPad/ProjectSolutionPad.cs:68 
  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
  at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00038] in /private/tmp/source-mono-4.4.0-c7sr0/bockbuild-mono-4.4.0-branch-c7sr0/profiles/mono-mac-xamarin/build-root/mono-x86/mcs/class/corlib/System.Reflection/MonoMethod.cs:295 
  --- End of inner exception stack trace ---
  at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x0004d] in /private/tmp/source-mono-4.4.0-c7sr0/bockbuild-mono-4.4.0-branch-c7sr0/profiles/mono-mac-xamarin/build-root/mono-x86/mcs/class/corlib/System.Reflection/MonoMethod.cs:303 
  at System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) [0x00000] in /private/tmp/source-mono-4.4.0-c7sr0/bockbuild-mono-4.4.0-branch-c7sr0/profiles/mono-mac-xamarin/build-root/mono-x86/external/referencesource/mscorlib/system/reflection/methodbase.cs:229 
  at System.Delegate.DynamicInvokeImpl (System.Object[] args) [0x00107] in /private/tmp/source-mono-4.4.0-c7sr0/bockbuild-mono-4.4.0-branch-c7sr0/profiles/mono-mac-xamarin/build-root/mono-x86/mcs/class/corlib/System/Delegate.cs:461 
  at System.MulticastDelegate.DynamicInvokeImpl (System.Object[] args) [0x0000b] in /private/tmp/source-mono-4.4.0-c7sr0/bockbuild-mono-4.4.0-branch-c7sr0/profiles/mono-mac-xamarin/build-root/mono-x86/mcs/class/corlib/System/MulticastDelegate.cs:67 
  at System.Delegate.DynamicInvoke (System.Object[] args) [0x00000] in /private/tmp/source-mono-4.4.0-c7sr0/bockbuild-mono-4.4.0-branch-c7sr0/profiles/mono-mac-xamarin/build-root/mono-x86/mcs/class/corlib/System/Delegate.cs:406 
  at System.MulticastDelegate.DynamicInvokeImpl (System.Object[] args) [0x0001e] in /private/tmp/source-mono-4.4.0-c7sr0/bockbuild-mono-4.4.0-branch-c7sr0/profiles/mono-mac-xamarin/build-root/mono-x86/mcs/class/corlib/System/MulticastDelegate.cs:72 
  at System.Delegate.DynamicInvoke (System.Object[] args) [0x00000] in /private/tmp/source-mono-4.4.0-c7sr0/bockbuild-mono-4.4.0-branch-c7sr0/profiles/mono-mac-xamarin/build-root/mono-x86/mcs/class/corlib/System/Delegate.cs:406 
  at GLib.Signal.ClosureInvokedCB (System.Object o, GLib.ClosureInvokedArgs args) <0x11edf1cd0 + 0x00172> in <filename unknown>:0 
  at GLib.SignalClosure.Invoke (GLib.ClosureInvokedArgs args) <0x11edf1c90 + 0x0002f> in <filename unknown>:0 
  at GLib.SignalClosure.MarshalCallback (IntPtr raw_closure, IntPtr return_val, UInt32 n_param_vals, IntPtr param_values, IntPtr invocation_hint, IntPtr marshal_data) <0x11edf0f70 + 0x002fb> in <filename unknown>:0 
ERROR [2016-07-07 18:48:33Z]: Exception while loading user solution preferences.
System.NullReferenceException: Object reference not set to an instance of an object
  at OmniSharp.Dnx.DnxProjectSystem.ChangeConfiguration (System.String config) <0x12dc98610 + 0x00340> in <filename unknown>:0 
  at MonoDevelop.Dnx.DnxProjectService.ActiveConfigurationChanged (System.Object sender, System.EventArgs e) <0x1269e3800 + 0x00072> in <filename unknown>:0 
  at (wrapper delegate-invoke) <Module>:invoke_void_object_EventArgs (object,System.EventArgs)
  at MonoDevelop.Ide.RootWorkspace.OnActiveConfigurationChanged () [0x0000b] in /Users/builder/data/lanes/3422/e879ce52/source/monodevelop/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide/RootWorkspace.cs:108 
  at MonoDevelop.Ide.RootWorkspace+<RestoreWorkspacePreferences>c__async7.MoveNext () [0x000e7] in /Users/builder/data/lanes/3422/e879ce52/source/monodevelop/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide/RootWorkspace.cs:600