Fix GUI json load.

[carolhmj]

Related issue: https://forum.babylonjs.com/t/gui-editor-will-generate-error-if-save-locally/34853/2

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Snapshot stored with reference name:
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Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/13120/merge/index.html

To test a playground add it to the URL, for example:

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Links to test babylon tools with this snapshot:

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https://sandbox.babylonjs.com/?snapshot=refs/pull/13120/merge
https://gui.babylonjs.com/?snapshot=refs/pull/13120/merge
https://nme.babylonjs.com/?snapshot=refs/pull/13120/merge

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Visualization tests for webgl1 have failed. If some tests failed because the snapshots do not match, the report can be found at

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[sebavan]

@carolhmj looks like this broke some tests ?

[carolhmj]

@carolhmj looks like this broke some tests ?

woops I forgot to tell the tests to wait for a few frames before screenshot, this should be fixed now 🤣