fix(proxy-state-tree): avoid returning spliced array on dipsoing onMutation

Author: christianalfoni

packages/node_modules/proxy-state-tree/src/index.ts

@@ -135,8 +135,9 @@ export class ProxyStateTree<T extends object> implements IProxyStateTree<T> {
   onMutation(callback: IMutationCallback) {
     this.mutationCallbacks.push(callback)
 
-    return () =>
+    return () => {
       this.mutationCallbacks.splice(this.mutationCallbacks.indexOf(callback), 1)
+    }
   }
   forceFlush() {
     const emptyMutations = []