🐛 Fix: Avoid a exception / crash on a bad cast, also comment around memory leaking

Signed-off-by: Mike Mahony <mikemahony@pixar.com>

Author: mikemahony

Sources/objc/opentimelineio.mm

@@ -730,6 +730,9 @@ CxxTimeRange timeline_range_of_child(CxxRetainer* self, CxxRetainer* child, CxxE
 NSArray* timeline_find_clips(CxxRetainer* self, CxxErrorStruct* cxxErr) {
     auto array = [NSMutableArray new];
     _AutoErrorHandler aeh(cxxErr);
+    // find_clips() returns pointers to Clips owned by the Timeline's composition
+    // hierarchy. NSValue wraps them for transport to Swift, where findOrCreate()
+    // creates properly-retained wrappers. Same pattern as composition_children_in_range().
     for (auto t: SO_cast<otio::Timeline>(self)->find_clips(&aeh.error_status)) {
         [array addObject: [NSValue valueWithPointer: t]];
     }

Sources/swift/Timeline.swift

@@ -82,7 +82,9 @@ public class Timeline : SerializableObjectWithMetadata {
         var result = [Clip]()
         for child in children_array {
             if let nsptr = child as? NSValue, let cxxPtr = nsptr.pointerValue {
-                result.append(SerializableObject.findOrCreate(cxxPtr: cxxPtr) as! Clip)
+                if let clip = SerializableObject.findOrCreate(cxxPtr: cxxPtr) as? Clip {
+                    result.append(clip)
+                }
             }
         }
         return result