libgimp: fix #11813 CRITICAL on NULL GimpObjectArray to plugin

We allow passing NULL gvalue for an empty GimpObjectArray, for convenience.
For example to plugin sel2path which doesn't use drawables.
But crossing the wire, create an empty GimpObjectArray
having an arbitrary type for elements.

libgimp/gimpgpparams-body.c

@@ -661,7 +661,9 @@ gimp_gp_param_to_value (gpointer        gimp,
 {
   g_return_if_fail (param != NULL);
   g_return_if_fail (value != NULL);
+  /* pspec is nullable. */
 
+  /* See also changing of types by caller. */
   if (type == G_TYPE_NONE || type == G_TYPE_INVALID)
     {
       type = g_type_from_name (param->type_name);
@@ -835,20 +837,35 @@ gimp_gp_param_to_value (gpointer        gimp,
     }
   else if (GIMP_VALUE_HOLDS_OBJECT_ARRAY (value))
     {
-      GType     object_type;
+      GType     object_type = G_TYPE_INVALID;
       GObject **objects;
       gint      i;
 
-      if (param->data.d_id_array.type_name == NULL)
+      /* Get type of the array elements. */
+      if (param->data.d_id_array.type_name != NULL)
+        {
+          /* An empty array from a NULL has an arbitrary type_name set earlier. */
+          object_type = g_type_from_name (param->data.d_id_array.type_name);
+        }
+      else if (pspec != NULL)
         {
-          g_return_if_fail (param->data.d_id_array.size == 0 && pspec != NULL);
           object_type = GIMP_PARAM_SPEC_OBJECT_ARRAY (pspec)->object_type;
         }
-      else
+      /* Else object-type is G_TYPE_INVALID*/
+
+      /* GimpObjectArray requires declared element type derived from GObject.
+       * Even when empty.
+       * When not, return without setting the gvalue.
+       * Not necessarily an error, when the plugin does not use the gvalue.
+       */
+      if (!g_type_is_a (object_type, G_TYPE_OBJECT))
         {
-          object_type = g_type_from_name (param->data.d_id_array.type_name);
+          g_warning ("%s returning NULL for GimpObjectArray", G_STRFUNC);
+          return;
         }
 
+      /* size might be zero. */
+
       objects = g_new (GObject *, param->data.d_id_array.size);
 
       for (i = 0; i < param->data.d_id_array.size; i++)
@@ -876,6 +893,9 @@ gimp_gp_param_to_value (gpointer        gimp,
             g_object_ref (objects[i]);
         }
 
+      /* Even when size is zero, set gvalue to an empty GimpObjectArray object,
+       * having a valid but possibly arbitrary type of its elements.
+       */
       gimp_value_take_object_array (value, object_type, objects,
                                     param->data.d_id_array.size);
     }
@@ -1277,8 +1297,18 @@ gimp_value_to_gp_param (const GValue *value,
         }
       else
         {
+          /* GValue intended to hold GimpObjectArray is NULL.
+           * For convenience, we allow this, meaning empty.
+           */
           param->data.d_id_array.size = 0;
           param->data.d_id_array.data = NULL;
+          /* Arbitrarily say elements are type Drawable.
+           * We must have a type to create an empty GimpObjectArray.
+           */
+          if (full_copy)
+            param->data.d_id_array.type_name = g_strdup ("GimpDrawable");
+          else
+            param->data.d_id_array.type_name = "GimpDrawable";
         }
     }
   else if (GIMP_VALUE_HOLDS_IMAGE (value))