Add error when reading invalid reference

Previously it would just return undefined which would throw an error when attempting to use the object later on. Now you’re able to demangle the stack trace to find exactly where you’re misusing a ref.

Author: j-f1

Runtime/src/index.ts

@@ -119,7 +119,11 @@ class SwiftRuntimeHeap {
     }
 
     referenceHeap(ref: ref) {
-        return this._heapValueById.get(ref)
+        const value = this._heapValueById.get(ref)
+        if (value === undefined) {
+            throw new ReferenceError("Attempted to read invalid reference " + ref)
+        }
+        return value
     }
 }