Fix T100850: Regression: Gpencil: crash on material color and other properties change.

rBcd49fee74114 forgot to add proper support for calls to `ntreeAddTree`
with a NULL Main (which should generate `NO_MAIN` ntrees).

source/blender/blenkernel/intern/node.cc

@@ -2569,7 +2569,7 @@ static bNodeTree *ntreeAddTree_do(
    * node groups and other tree types are created as library data.
    */
   int flag = 0;
-  if (is_embedded) {
+  if (is_embedded || bmain == nullptr) {
     flag |= LIB_ID_CREATE_NO_MAIN;
   }
   bNodeTree *ntree = (bNodeTree *)BKE_libblock_alloc(bmain, ID_NT, name, flag);