(in) (nullable) GError * parameter not accepted as None

So we've got an API which takes a GError * as (in) argument, and we want this to be optional (actually most of the time, you should not run it with a NULL). So we added (nullable) (cf. also source).

/**
 * gimp_procedure_new_return_values:
 * @procedure: the #GimpProcedure.
 * @status:    the success status of the procedure run.
 * @error:     (in) (nullable) (transfer full):
 *             an optional #GError. This parameter should be set if
 *             @status is either #GIMP_PDB_EXECUTION_ERROR or
 *             #GIMP_PDB_CALLING_ERROR.
 *
 * Format the expected return values from procedures, using the return
 * values set with gimp_procedure_add_return_value().
 *
 * Returns: the expected #GimpValueArray as could be returned by a
 *          #GimpRunFunc.
 */

Yet calling it from Python generates an error:

Traceback (most recent call last):
  File "/home/jehan/.local/share/crossroad/roads/native/gimp/lib/gimp/2.99/plug-ins/palette-to-gradient/palette-to-gradient.py", line 115, in run_palette_to_gradient
    return procedure.new_return_values(Gimp.PDBStatusType.SUCCESS, None)
TypeError: Must be GLib.Error, not NoneType

Apparently there might be special casing in pygobject forbidding a GError* to end up NULL?

Cf. also the Gimp-3.0.gir file which looks good:

      <method name="new_return_values"
              c:identifier="gimp_procedure_new_return_values">
        <doc xml:space="preserve"
             filename="src/gimp/libgimp/gimpprocedure.c"
             line="475">Format the expected return values from procedures, using the return
values set with gimp_procedure_add_return_value().</doc>
        <source-position filename="src/gimp/libgimp/gimpprocedure.h"
                         line="114"/>
        <return-value transfer-ownership="full">                
          <doc xml:space="preserve"
               filename="src/gimp/libgimp/gimpprocedure.c"
               line="487">the expected #GimpValueArray as could be returned by a
         #GimpRunFunc.</doc>
          <type name="ValueArray" c:type="GimpValueArray*"/>
        </return-value>
        <parameters>
          <instance-parameter name="procedure" transfer-ownership="none">
            <doc xml:space="preserve"
                 filename="src/gimp/libgimp/gimpprocedure.c"
                 line="477">the #GimpProcedure.</doc>
            <type name="Procedure" c:type="GimpProcedure*"/>
          </instance-parameter>
          <parameter name="status" transfer-ownership="none">
            <doc xml:space="preserve"
                 filename="src/gimp/libgimp/gimpprocedure.c"
                 line="478">the success status of the procedure run.</doc>
            <type name="PDBStatusType" c:type="GimpPDBStatusType"/>
          </parameter>
          <parameter name="error"
                     transfer-ownership="full"
                     nullable="1"
                     allow-none="1">
            <doc xml:space="preserve"
                 filename="src/gimp/libgimp/gimpprocedure.c"
                 line="479">
            an optional #GError. This parameter should be set if
            @status is either #GIMP_PDB_EXECUTION_ERROR or
            #GIMP_PDB_CALLING_ERROR.</doc>
            <type name="GLib.Error" c:type="GError*"/>
          </parameter>
        </parameters> 
      </method>