Add assertion for creating media pool item if not found

Asserts creation of media pool item when not found for specified files. This ensures proper handling and error messaging in such cases.

client/ayon_core/hosts/resolve/api/plugin.py

@@ -405,6 +405,11 @@ class ClipLoader:
             files,
             self.active_bin
         )
+
+        assert media_pool_item, AssertionError(
+            "Cannot create media pool item for files: `{}`".format(files)
+        )
+
         _clip_property = media_pool_item.GetClipProperty
         source_in = int(_clip_property("Start"))
         source_out = int(_clip_property("End"))