@changsi-an If you pass undefined for the folder parameter to vscode.debug.startDebugging, the debug configuration myConfig must not contain any variables (like ${workspaceFolder}) because variables are resolved against the folder parameter (which is undefined in your case).
So you must pass a folder to vscode.debug.startDebugging if you have a folder. Only in a non-folder setup it makes sense to pass undefined (and in this case a variable like ${workspaceFolder}does not make sense at all).

@isidorn we should fail more gracefully in this case, e.g. by not substituting any variables at all.

@weinand I don't have any variables in my myConfig object. All plain strings.

@isidorn most likely passing undefined to startDebugging results in the error from above in https://github.com/Microsoft/vscode/blob/1d632ec12cd4ec0fe4f8badfbb743d5bd2807cda/src/vs/workbench/parts/debug/electron-browser/debugService.ts#L905
because the "root" passed to "doCreateProcess" is "undefined".

@changsi-an good catch, thanks. You can try it out in vscode insiders in 4 days and let us know how ti works.
@weinand thanks for pointing out the issue

vscode.debug.startDebugging(undefined, 'Launch Program'); => nothing happens

vscode.debug.startDebugging(vscode.workspace.workspaceFolders[0], 'Launch Program'); => starts debugging

And I get this error in devtools: Configuration 'Launch Program' is missing in 'launch.json'.

@roblourens
What you are seeing is the expected behaviour.
if you pass undefined for the workspace folder, then no 'launch.json' and consequently no named launch config can be found. However, in this case it is perfectly fine to pass an launch config object to startDebugging.

The error message Configuration 'Launch Program' is missing in 'launch.json'is misleading because the real problem is that there is no 'launch.json' at all.

@isidorn can we make this clearer?

@weinand sure, I have improved the error message to be more precise in the case the launch.json does not exist. Now it simply says "'launch.json' does not exist."

Oh, I get it.

Is the bug fix released yet? I am still hitting "uri" of undefined error in 1.18.1

No, it's in Insiders or the next stable release.