flatten method crashes MATLAB in >=2020a

[k1o0]

Describe the bug Posting a value to a node upstream of flatten causes a catastrophic crash in MATLAB.

To Reproduce

a = sig.test.create;
a_flat = a.flatten;
a.post(rand) % BOOM!

Desktop (please complete the following information):

• Code version (e.g. 2.3.1) - Rigbox v2.6.1, signals v1.3.1
• MATLAB version - 9.8.0 and 9.9.0

[k1o0]

2020a release notes on the subject