Stress is null: cannot continue optimization

[jidel]

Reproducable:

1. Select some nodes
2. disconnect nodes
3. optimize
4. re-enable nodes

Result: Stress is null, cannot continue optimization

[rohankoid]

The case mentioned above is due to wrong use of re-enable feature.
However, Stress is null in other cases is a valid condition and not a system error.

[idrissou]

@jidel: This bug seems to disappear as soon as you move nodes (step 5) after re-enabling them and optimizing again (step 6). Everything including the stress returns back to normal!