flatten method crashes MATLAB in >=2020a

**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