Fix error when filtering in Outliner "Blender File" mode with libraries

Steps to reproduce were:
* Load factory settings
* Link any library
* Change to "Blender File" display mode in Outliner
* Enable filtering
* -> Assert fails

This may have had further side-effects for linked IDs.
Checked with Bastien, the NULL-check seems reasonable. It was initially there
but removed in d74f9c4b7b4f.

Fine to backport.

1 files changed, 4 insertions, 2 deletions

diff --git a/source/blender/editors/space_outliner/outliner_tree.c b/source/blender/editors/space_outliner/outliner_tree.c
index f07bb800eca..bc272431e3a 100644
--- a/source/blender/editors/space_outliner/outliner_tree.c
+++ b/source/blender/editors/space_outliner/outliner_tree.c
@@ -2356,8 +2356,10 @@ void outliner_build_tree(
 
     for (lib = mainvar->libraries.first; lib; lib = lib->id.next) {
       ten = outliner_add_library_contents(mainvar, soops, &soops->tree, lib);
-      BLI_assert(ten != NULL);
-      lib->id.newid = (ID *)ten;
+      /* NULL-check matters, due to filtering there may not be a new element. */
+      if (ten) {
+        lib->id.newid = (ID *)ten;
+      }
     }
     /* make hierarchy */
     ten = soops->tree.first;