Fix bug in Python "example.wbt" world

[MNikoliCC]

Description
When running the simulation this error message appears: Error: wb_supervisor_field_get_type() called with invalid 'field' argument.

Related Issues
Fixes #6707

Tasks

• Establish where the error comes from
• Fix the error

Additional Information
I suspect that the bug is related to PR #6613.

webots/src/controller/c/supervisor.c

Lines 1398 to 1422 in 37f0c14

	static bool check_field(WbFieldRef f, const char *function, WbFieldType type, bool check_type, int *index, bool is_importing,
	bool check_type_internal) {
	if (!robot_check_supervisor(function))
	return false;
	if (!f) {
	if (!robot_is_quitting())
	fprintf(stderr, "Error: %s() called with NULL 'field' argument.\n", function);
	return false;
	}
	// check field reference is valid
	WbFieldStruct *field = field_list;
	bool found = false;
	while (field) {
	if (field == f) {
	found = true;
	break;
	}
	field = field->next;
	}
	if (!found) {
	fprintf(stderr, "Error: %s() called with invalid 'field' argument.\n", function);
	return false;
	}

The check_field failed, meaning the field reference (WbFieldRef) was not found in the field_list.
That means that the field_list is not updated correctly, or the field reference f was not properly retreived from wb_supervisor_field_get_type method.

[lukicdarkoo]

Looks good, thank you