Fix some NME bugs.

[carolhmj]

1 - When dragging a unique block to the graph twice and closing the error message, for each subsequent block added (even the non-unique ones), the error message was still appearing.
2 - When using the PerturbNormal block without the connected normalColor input, the following error message appeared:

[azure-pipelines]

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[azure-pipelines]

Snapshot stored with reference name:
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Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/12951/merge/index.html

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https://sandbox.babylonjs.com/?snapshot=refs/pull/12951/merge
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