Incorrect conversions from null to undefined in new RPC

[colin-grant-work]

Bug Description:

In certain cases, null values from the backend are appearing on the frontend as undefined.

Steps to Reproduce:

1. Install and activate the vscode-lldb plugin.
2. See error mentioned here: Failure to activate because of problem in a method startRpcServer.

The error does not occur in 1.26 (without new RPC) but does in 1.27 with new RPC.

---

You can also use this plugin (source). Assuming you have not set the preference peaSoup.secretIngredients, running the command Access a (probably) non-existent preference should show a toast with the default value null, but on master it shows [].

Additional Information

The relevant preference is defined here with a default of null. However, when the plugin's data is transmitted to the frontend, that null becomes undefined, and we enter this code:

theia/packages/core/src/browser/preferences/preference-contribution.ts

Lines 248 to 271 in 84b317a

	getDefaultValue(property: PreferenceItem): JSONValue {
	if (property.defaultValue !== undefined) {
	return property.defaultValue;
	}
	if (property.default !== undefined) {
	return property.default;
	}
	const type = Array.isArray(property.type) ? property.type[0] : property.type;
	switch (type) {
	case 'boolean':
	return false;
	case 'integer':
	case 'number':
	return 0;
	case 'string':
	return '';
	case 'array':
	return [];
	case 'object':
	return {};
	}
	// eslint-disable-next-line no-null/no-null
	return null;
	}

Because the preference is typed as ["object", "null"], that method returns a default of {} rather than the configured default of null.

I have confirmed that null is present here, but becomes undefined when received here.

• Operating System: MacOS 12.4
• Theia Version: 1.27.0

[colin-grant-work]

Attn: @JonasHelming, @tortmayr

[JonasHelming]

@tortmayr