Omnisharp fails to Start with NetCore 5.0 RC2

[mdmoura]

Issue Description

When working on an NetCore 5.0 class library Omnisharp fails to load. The project file is:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net5.0</TargetFramework>
  </PropertyGroup>
</Project>

OmniSharp log

Starting OmniSharp server at 10/22/2020, 12:04:45 PM Target: /Users/mky/mky.Project/mky.Project.mky.Core/src/mky.Core

OmniSharp server started with Mono 6.12.0.
Path: /Users/mky/.vscode/extensions/ms-dotnettools.csharp-1.23.4/.omnisharp/1.37.4-beta.2/omnisharp/OmniSharp.exe
PID: 4228

Starting OmniSharp on MacOS 10.15.7 (x64)
info: OmniSharp.Services.DotNetCliService
DotNetPath set to dotnet
info: OmniSharp.MSBuild.Discovery.MSBuildLocator
Located 2 MSBuild instance(s)
1: Mono 16.6.0 - "/Library/Frameworks/Mono.framework/Versions/6.12.0/lib/mono/msbuild/Current/bin"
2: StandAlone 16.8.0 - "/Users/mky/.vscode/extensions/ms-dotnettools.csharp-1.23.4/.omnisharp/1.37.4-beta.2/omnisharp/.msbuild/Current/Bin"
info: OmniSharp.MSBuild.Discovery.MSBuildLocator
Registered MSBuild instance: Mono 16.6.0 - "/Library/Frameworks/Mono.framework/Versions/6.12.0/lib/mono/msbuild/Current/bin"
CscToolExe = csc.exe
CscToolPath = /Users/mky/.vscode/extensions/ms-dotnettools.csharp-1.23.4/.omnisharp/1.37.4-beta.2/omnisharp/.msbuild/Current/Bin/Roslyn
info: OmniSharp.Cake.CakeProjectSystem
Detecting Cake files in '/Users/mky/mky.Project/mky.Project.mky.Core/src/mky.Core'.
info: OmniSharp.Cake.CakeProjectSystem
Could not find any Cake files
info: OmniSharp.MSBuild.ProjectSystem
No solution files found in '/Users/mky/mky.Project/mky.Project.mky.Core/src/mky.Core'
info: OmniSharp.MSBuild.ProjectManager
Queue project update for '/Users/mky/mky.Project/mky.Project.mky.Core/src/mky.Core/mky.Core.csproj'
info: OmniSharp.Script.ScriptProjectSystem
Detecting CSX files in '/Users/mky/mky.Project/mky.Project.mky.Core/src/mky.Core'.
info: OmniSharp.Script.ScriptProjectSystem
Could not find any CSX files
info: OmniSharp.WorkspaceInitializer
Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.CSharpFormattingWorkspaceOptionsProvider, Order: 0
info: OmniSharp.MSBuild.ProjectManager
Loading project: /Users/mky/mky.Project/mky.Project.mky.Core/src/mky.Core/mky.Core.csproj
info: OmniSharp.WorkspaceInitializer
Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.Completion.CompletionOptionsProvider, Order: 0
info: OmniSharp.WorkspaceInitializer
Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.RenameWorkspaceOptionsProvider, Order: 100
info: OmniSharp.WorkspaceInitializer
Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.ImplementTypeWorkspaceOptionsProvider, Order: 110
info: OmniSharp.WorkspaceInitializer
Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.BlockStructureWorkspaceOptionsProvider, Order: 140
info: OmniSharp.WorkspaceInitializer
Configuration finished.
info: OmniSharp.Stdio.Host
Omnisharp server running using Stdio at location '/Users/mky/mky.Project/mky.Project.mky.Core/src/mky.Core' on host 3828.
fail: OmniSharp.MSBuild.ProjectLoader
The reference assemblies for .NETFramework,Version=v5.0 were not found. To resolve this, install the Developer Pack (SDK/Targeting Pack) for this framework version or retarget your application. You can download .NET Framework Developer Packs at https://aka.ms/msbuild/developerpacks
[warn]: OmniSharp.MSBuild.ProjectManager
Failed to load project file '/Users/mky/mky.Project/mky.Project.mky.Core/src/mky.Core/mky.Core.csproj'.
/Users/mky/mky.Project/mky.Project.mky.Core/src/mky.Core/mky.Core.csproj
/Library/Frameworks/Mono.framework/Versions/6.12.0/lib/mono/msbuild/Current/bin/Microsoft.Common.CurrentVersion.targets(1193,5): Error: The reference assemblies for .NETFramework,Version=v5.0 were not found. To resolve this, install the Developer Pack (SDK/Targeting Pack) for this framework version or retarget your application. You can download .NET Framework Developer Packs at https://aka.ms/msbuild/developerpacks

Attempted to update project that is not loaded: /Users/mky/mky.Project/mky.Project.mky.Core/src/mky.Core/mky.Core.csproj

Environment information

VSCode version: 1.50.1
C# Extension: 1.23.4

Mono Information

OmniSharp using global mono :6.12.0

Dotnet Information

.NET SDK (reflecting any global.json): Version: 5.0.100-rc.2.20479.15 Commit: da7dfa8840

Runtime Environment:
OS Name: Mac OS X
OS Version: 10.15
OS Platform: Darwin
RID: osx.10.15-x64
Base Path: /usr/local/share/dotnet/sdk/5.0.100-rc.2.20479.15/

Host (useful for support):
Version: 5.0.0-rc.2.20475.5
Commit: c5a3f49c88

.NET SDKs installed:
5.0.100-rc.2.20479.15 [/usr/local/share/dotnet/sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 5.0.0-rc.2.20475.17 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 5.0.0-rc.2.20475.5 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]

To install additional .NET runtimes or SDKs:
https://aka.ms/dotnet-download

Visual Studio Code Extensions

Extension	Author	Version
Angular2	johnpapa	9.1.2
azure-account	ms-vscode	0.9.3
azure-pipelines	ms-azure-devops	1.177.0
csharp	ms-dotnettools	1.23.4
markdown-preview-enhanced	shd101wyy	0.5.13
mdmath	goessner	2.5.1
ml-net-model-builder	franzssilva	0.1.1
mssql	ms-mssql	1.9.0
remote-containers	ms-vscode-remote	0.145.1
typescript-hero	rbbit	3.0.0
vetur	octref	0.28.0
vscode-docker	ms-azuretools	1.7.0
vscode-mjml	attilabuti	1.6.0
vscode-sqlite	alexcvzz	0.9.0
vscode-yaml	redhat	0.11.1

[filipw]

sorry for the trouble. This is a duplicate. Please see here for a solution #4134 (comment)