Try a different approach for el_refs

`CompoundPhaseDiagram` contains `DummySpecies` not `Element`

pymatgen/analysis/phase_diagram.py

@@ -366,7 +366,7 @@ def __init__(self, entries, elements=None, *, computed_data=None):
         self.all_entries = computed_data["all_entries"]
         self.qhull_data = computed_data["qhull_data"]
         self.dim = computed_data["dim"]
-        self.el_refs = {Element(el): ref for el, ref in computed_data["el_refs"].items()}
+        self.el_refs = dict(computed_data["el_refs"])
         self.qhull_entries = computed_data["qhull_entries"]
         self.stable_entries = set(self.qhull_entries[i] for i in set(itertools.chain(*self.facets)))
 
@@ -461,7 +461,8 @@ def _compute(self):
             all_entries=all_entries,
             qhull_data=qhull_data,
             dim=dim,
-            el_refs={str(el): ref for el, ref in el_refs.items()},
+            # Dictionary with Element keys is not JSON-serializable
+            el_refs=list(el_refs.items()),
             qhull_entries=qhull_entries,
         )