Thread Demo callback failed

[GatoAmarilloBicolor]

Godot version:

Alpha 3
3.1 mono
linux mint
OS/device including version:

r7 430 amd

Issue description:

The debug message come after i try to play the thread demo

Steps to reproduce:
download the trhead demo on the store

Minimal reproduction project:

./Godot_v3.1-alpha3_mono_x11.64
OpenGL ES 3.0 Renderer: AMD HAINAN (DRM 2.50.0 / 4.15.0-42-generic, LLVM 6.0.0)
Editing project: /home/cmkat/app/Godot/alfa/demos/threads (::home::cmkat::app::Godot::alfa::demos::threads)
OpenGL ES 3.0 Renderer: AMD HAINAN (DRM 2.50.0 / 4.15.0-42-generic, LLVM 6.0.0)
Godot_v3.1-alpha3_mono_x11_64> ERROR: set_item_checked: Index p_idx=-1 out of size (items.size()=2)
At: scene/gui/popup_menu.cpp:763.
ERROR: set_item_checked: Index p_idx=-1 out of size (items.size()=2)
At: scene/gui/popup_menu.cpp:763.

Unhandled Exception:
System.IO.FileNotFoundException: Could not find file "/home/cmkat/app/Godot/alfa/demos/threads/Loading in a Thread.csproj"
File name: '/home/cmkat/app/Godot/alfa/demos/threads/Loading in a Thread.csproj'
at System.IO.FileStream..ctor (System.String path, System.IO.FileMode mode, System.IO.FileAccess access, System.IO.FileShare share, System.Int32 bufferSize, System.Boolean anonymous, System.IO.FileOptions options) [0x0019e] in :0
at System.IO.FileStream..ctor (System.String path, System.IO.FileMode mode, System.IO.FileAccess access, System.IO.FileShare share, System.Int32 bufferSize) [0x00000] in :0
at (wrapper remoting-invoke-with-check) System.IO.FileStream..ctor(string,System.IO.FileMode,System.IO.FileAccess,System.IO.FileShare,int)
at System.Xml.XmlDownloadManager.GetStream (System.Uri uri, System.Net.ICredentials credentials, System.Net.IWebProxy proxy, System.Net.Cache.RequestCachePolicy cachePolicy) [0x00018] in :0
at System.Xml.XmlUrlResolver.GetEntity (System.Uri absoluteUri, System.String role, System.Type ofObjectToReturn) [0x00032] in :0
at System.Xml.XmlTextReaderImpl.FinishInitUriString () [0x0004c] in :0
at System.Xml.XmlTextReaderImpl..ctor (System.String uriStr, System.Xml.XmlReaderSettings settings, System.Xml.XmlParserContext context, System.Xml.XmlResolver uriResolver) [0x000c7] in :0
at System.Xml.XmlReaderSettings.CreateReader (System.String inputUri, System.Xml.XmlParserContext inputContext) [0x0003b] in :0
at System.Xml.XmlReader.Create (System.String inputUri, System.Xml.XmlReaderSettings settings, System.Xml.XmlParserContext inputContext) [0x0000a] in :0
at System.Xml.XmlReader.Create (System.String inputUri) [0x00000] in :0
at Microsoft.Build.Construction.ProjectRootElement.Open (System.String path, Microsoft.Build.Evaluation.ProjectCollection projectCollection) [0x00008] in <47af3aeb89234d58acd376f80276c92c>:0
at Microsoft.Build.Construction.ProjectRootElement.Open (System.String path) [0x00006] in <47af3aeb89234d58acd376f80276c92c>:0
at GodotSharpTools.Project.ProjectUtils.GetIncludeFiles (System.String projectPath, System.String itemType) [0x00029] in <1a22a837331443c0afd658140fa01e9a>:0
ERROR: generate_scripts_metadata: Method/Function Failed, returning: FAILED
At: modules/mono/editor/csharp_project.cpp:155.
ERROR: editor_build_callback: Condition ' metadata_err != OK ' is true. returned: false
At: modules/mono/editor/godotsharp_builds.cpp:388.
ERROR: call_build: A Godot Engine build callback failed.
At: editor/editor_node.cpp:4494.

[akien-mga]

Seems to work fine in 3.1 beta 6, both with Mono and without.