Fix possibly missing parent update when setting null values

CHANGELOG.md

@@ -8,6 +8,7 @@
 ### Fixed
 * <How do the end-user experience this issue? what was the impact?> ([#????](https://github.com/realm/realm-core/issues/????), since v?.?.?)
 * Query parser would not accept "in" as a property name ([#5312](https://github.com/realm/realm-core/issues/5312))
+* Application would sometimes crash with exceptions like 'KeyNotFound' or assertion "has_refs()". Other issues indicating file corruption may also be fixed by this. The one mentioned here is the one that lead to solving the problem. ([#5283](https://github.com/realm/realm-core/issues/5283), since v6.0.0)
 
 ### Breaking changes
 * SchemaMode::ResetFile renamed to SchemaMode::SoftResetFile.

src/realm/obj.cpp

@@ -1774,7 +1774,7 @@ inline void nullify_set(Obj& obj, ColKey origin_col_key, T target)
 
 void Obj::nullify_link(ColKey origin_col_key, ObjLink target_link)
 {
-    ensure_writeable();
+    // ensure_writeable();
 
     ColKey::Idx origin_col_ndx = origin_col_key.get_index();
     Allocator& alloc = get_alloc();
@@ -2161,6 +2161,8 @@ inline void Obj::do_set_null(ColKey col_key)
     values.set_parent(&fields, col_ndx.val + 1);
     values.init_from_parent();
     values.set_null(m_row_ndx);
+
+    sync(fields);
 }
 
 template <>
@@ -2178,6 +2180,8 @@ inline void Obj::do_set_null<ArrayString>(ColKey col_key)
     values.set_spec(const_cast<Spec*>(&get_spec()), spec_ndx);
     values.init_from_parent();
     values.set_null(m_row_ndx);
+
+    sync(fields);
 }
 
 Obj& Obj::set_null(ColKey col_key, bool is_default)
@@ -2194,7 +2198,6 @@ Obj& Obj::set_null(ColKey col_key, bool is_default)
         }
 
         update_if_needed();
-        ensure_writeable();
 
         StringIndex* index = m_table->get_search_index(col_key);
         if (index && !m_key.is_unresolved()) {