Error with Nullius Rocket Recipes

[KiwiHawk]

Error when adding a rocket launch product recipe from Nullius.

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Nullius hides the default rocket silo and creates it's own nullius-silo. It has a different fixed recipe nullius-rocket with category rocketry. The base game rocket-part recipe uses the category rocket-building.

Change Helmod, removing the hard coded reference to rocket-building.

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Limitations

In theory, there's nothing preventing a mod from having multiple enabled rocket silos - each with their own fixed recipe. This can be handled at a future stage, if it ever comes up.