Validation failure after creating a new line

[weigao-123]

Currently, if we create a new line using the web interface, by default, it is using the same bus for the source and target, and if the user directly exports this instance and then load it again, the json schema validation will fail:

{
    "instancePath": "/Transmission lines/l21/Target bus",
    "schemaPath": "#/definitions/TransmissionLines/additionalProperties/properties/Target%20bus/not",
    "keyword": "not",
    "params": {},
    "message": "must NOT be valid",
    "schema": {
        "const": {
            "$data": "1/Source bus"
        }
    },
    "parentSchema": {
        "type": "string",
        "minLength": 1,
        "not": {
            "const": {
                "$data": "1/Source bus"
            }
        }
    },
    "data": "b1"
}