Cleanup references to instance when disconnecting!

store.js

@@ -37,6 +37,20 @@ export const Store = function(superClass) {
       this._writeKey(key, value);
     }
 
+    disconnectedCallback() {
+      super.disconnectedCallback();
+      
+      // Clean up all references to this instance.
+      instancesPerKey.forEach((instances, key) => {
+        const remainingInstances = instances.filter(entry => entry.instance !== this);
+        if (remainingInstances.length === 0) {
+          instancesPerKey.delete(key);
+        } else {
+          instancesPerKey.set(key, remainingInstances);
+        }
+      });
+    }
+
     _addInstance(instance, key, targetProperty) {
       if (instancesPerKey.has(key) === false) {
         instancesPerKey.set(key, [ { instance, targetProperty } ]);