Interpreter freezes upon trying to call non-existing member functions

[MarioLiebisch]

Describe the bug Calling a non-existing member function in any lesson freezes the whole app with an uncaught runtime error popping up in the (browser) console: null function or function signature mismatch.

To Reproduce Steps to reproduce the bug:

1. Open any practice.
2. Define a new variable or use an existing one and call any non-existing member.
3. Click on Run.
4. The app freezes, triggering the runtime error.

Expected behavior The non-existing member should be noted instead of the app freezing.

Screenshots While this example is artificial, I've noticed it by making a typo using pop_font() rather than pop_front().

Information about your device (please complete the following information):

• Latest Google Chrome (stable)