Hi,

In the Relation Converter, method toFieldDefinition(), you use a string assignment in case of "$storageDef->dataInt2" equals 0.

$fieldDef->fieldTypeConstraints->fieldSettings['selectionRoot'] = $storageDef->dataInt2 === 0 ? '' : $storageDef->dataInt2;

But, it's an integer variable, so it makes an error.
I changed it for a ternary condition without string.

$fieldDef->fieldTypeConstraints->fieldSettings['selectionRoot'] = $storageDef->dataInt2 === 0 ? null : $storageDef->dataInt2;

Rgds,
C Vincent