In 1.26 we discovered that several extensions call the 'vscode.openFolder' command with invalid file URIs:

• NAVBaaS.git/out/commands/Go.js
  136,51: yield vscode.commands.executeCommand('vscode.openFolder', vscode.Uri.parse(gitRepoFolder));
• NAVBaaS.git/out/commands/openGitRepoFolder.js
  27,51: yield vscode.commands.executeCommand('vscode.openFolder', vscode.Uri.parse(gitRepoFolder));
• NAVBaaS.git/out/commands/openWorkspace.js
  27,51: yield vscode.commands.executeCommand('vscode.openFolder', vscode.Uri.parse(gitRepoFolder));

When creating a URI from a file path, always use vscode.URI.file(path), e.g vscode.URI.file('c:\\test')
Use vscode.URI.parse(uriString) when the input string is in the form scheme://authority/path

See microsoft/vscode#57267 for more information or for questions.