[BUG] Handle loading models with custom RDF prefixes

[atextor]

How: List of performed steps

Create a model that contains a custom RDF prefix, e.g.,

@prefix ex: <http://example.com#> .

What: Actual result vs expected result
Expected: Custom prefix is loaded, and is kept when the model is saved again
Actual: "Please wait until the model is loaded!" endlessly displayed, model is not loaded

Where: (e.g. component, version, url, your system info)
AME 5.6.1

Input (files/screenshots)

Output (files/screenshots)

Additional context (e.g. references)