Nodes: Fix T90233

- Fix default_value initialization of custom node tree interface:

  This was crashing when adding a custom interface socket to a tree.
  The node_socket_set_typeinfo function was called too early, creating a
  default float socket, which then doesn't match the socket type after
  changing to the custom type.

  The node_socket_set_typeinfo only allocates and initializes
  default_value when it isn't already set. That is because the function is
  used either when creating new sockets or to initialize typeinfo after
  loading files. So default_value has to be either null or has to be
  matching the current type already.

- Fix RNA flag for string return value of the valid_socket_type callback:

  String return values of registerable RNA functions need a
  PROP_THICK_WRAP flag since they don't have a fixed buffer to write into.

source/blender/blenkernel/intern/node.cc

@@ -1165,6 +1165,9 @@ static void node_set_typeinfo(const struct bContext *C,
   }
 }
 
+/* Warning: default_value must either be null or match the typeinfo at this point.
+ * This function is called both for initializing new sockets and after loading files.
+ */
 static void node_socket_set_typeinfo(bNodeTree *ntree,
                                      bNodeSocket *sock,
                                      bNodeSocketType *typeinfo)
@@ -3286,9 +3289,9 @@ static bNodeSocket *make_socket_interface(bNodeTree *ntree,
 
   bNodeSocket *sock = MEM_cnew<bNodeSocket>("socket template");
   BLI_strncpy(sock->idname, stype->idname, sizeof(sock->idname));
-  node_socket_set_typeinfo(ntree, sock, stype);
   sock->in_out = in_out;
   sock->type = SOCK_CUSTOM; /* int type undefined by default */
+  node_socket_set_typeinfo(ntree, sock, stype);
 
   /* assign new unique index */
   const int own_index = ntree->cur_index++;

source/blender/makesrna/intern/rna_nodetree.c

@@ -12441,7 +12441,7 @@ static void rna_def_nodetree(BlenderRNA *brna)
   RNA_def_function_flag(func, FUNC_NO_SELF | FUNC_REGISTER_OPTIONAL);
   parm = RNA_def_string(
       func, "idname", "NodeSocket", MAX_NAME, "Socket Type", "Identifier of the socket type");
-  RNA_def_parameter_flags(parm, PROP_NEVER_NULL, PARM_REQUIRED);
+  RNA_def_parameter_flags(parm, PROP_NEVER_NULL | PROP_THICK_WRAP, PARM_REQUIRED);
   RNA_def_function_return(func, RNA_def_boolean(func, "valid", false, "", ""));
 }