vscode-python: Unittesting not working: "Test framework undefined is not installed. Install?"

Environment data

• VS Code version: 1.30.2 (user setup)
• Extension version (available under the Extensions sidebar): Python 2018.12.1
• OS and version: Windows_NT x64 10.0.17134
• Python version (& distribution if applicable, e.g. Anaconda): Python 3.6 through Anaconda
• Type of virtual environment used (N/A | venv | virtualenv | conda | …): conda, venv…
• Relevant/affected Python packages and their versions: None

Expected behaviour

I try to run unit tests using the standard unittest framework following the steps outlined in this tutorial.

Actual behaviour

However, when discovering the unittests, I get this error message: “Test framework undefined is not installed. Install?”.

Steps to reproduce:

Same as in the tutorial.

Logs

Output for Python in the Output panel (View→Output, change the drop-down the upper-right of the Output panel to Python)

Starting Jedi Python language engine.
##########Linting Output - flake8##########
##########Linting Output - pylint##########

--------------------------------------------------------------------

Your code has been rated at 10.00/10 (previous run: 10.00/10, +0.00)



##########Linting Output - flake8##########
##########Linting Output - pylint##########

--------------------------------------------------------------------

Your code has been rated at 10.00/10 (previous run: 10.00/10, +0.00)

Output from Console under the Developer Tools panel (toggle Developer Tools on under Help; turn on source maps to make any tracebacks be useful by running Enable source map support for extension debugging)

[Extension Host] Python Extension: displayDiscoverStatus TypeError: Cannot read property 'uri' of undefined
	at module.exports.t.BaseTestManager.discoverTests.r.serviceContainer.get.discoverTests.then.catch.e (C:\Users\jmg\.vscode\extensions\ms-python.python-2018.12.1\out\client\extension.js:1:269713)
	at <anonymous>
	at process._tickCallback (internal/process/next_tick.js:188:7)
t.log @ console.ts:134
notificationsAlerts.ts:40 Test framework undefined is not installed. Install?
t.onDidNotificationChange @ notificationsAlerts.ts:40
notificationsAlerts.ts:40 Test discovery error, please check the configuration settings for the tests.