Theia: Impossible to run a task w/o a problem matcher specified

[azatsarynnyy]

Bug Description:

It's impossible to run a task w/o a problem matcher specified.

Steps to Reproduce:

1. Run any task w/o a problem marcher specified.
2. Theia should ask you what problem matcher to use.
3. Actually, the list of problem matcher is empty

Additional Information

• Operating System: Fedora
• Theia Version: 362e8f13e97a850387c103e8e7ee29d6e36b873e

[azatsarynnyy]

It works before this PR https://github.com/eclipse-theia/theia/pull/6921 changes, on commit 3f28503e754bbb4fa6534612af3d1ed6da3ed66a.
But it doesn't work after the mentioned PR, on commit 362e8f13e97a850387c103e8e7ee29d6e36b873e.

cc/ @akosyakov Somehow that PR broke the task problem matchers list. I see no errors in the logs.

[akosyakov]

@azatsarynnyy i will look at it tomorrow morning

Something is fishy here: https://github.com/eclipse-theia/theia/blob/362e8f13e97a850387c103e8e7ee29d6e36b873e/packages/core/src/browser/quick-open/quick-pick-service-impl.ts#L69

VS Code always shows the quick pick even if there are no items. I've fixed it and thought that the guard ensures that it does not break Theia extensions, but it does not seem to be the case. I will add an integration test for it.

[akosyakov]

that's even worse: try to change language or encoding of an editor, it just shows an empty quick pick.

[akosyakov]

@azatsarynnyy here is the fix: https://github.com/eclipse-theia/theia/pull/7804