return a promise if mappings alrady requested

src/storage/storageViewer.js

@@ -11,6 +11,7 @@ class StorageViewer {
   constructor (_context, _storageResolver, _traceManager) {
     this.context = _context
     this.storageResolver = _storageResolver
+    this.completeMapingsLocationPromise = null
     _traceManager.accumulateStorageChanges(this.context.stepIndex, this.context.address, {}, (error, storageChanges) => {
       if (!error) {
         this.storageChanges = storageChanges
@@ -71,9 +72,10 @@ class StorageViewer {
     * @param {Function} callback
     */
   async mappingsLocation () {
-    return new Promise((resolve, reject) => {
+    if (!this.completeMapingsLocationPromise) {
+      this.completeMapingsLocationPromise = new Promise((resolve, reject) => {
         if (this.completeMappingsLocation) {
-        return resolve(this.completeMappingsLocation)
+          return this.completeMappingsLocation
         }
         this.storageResolver.initialPreimagesMappings(this.context.tx, this.context.stepIndex, this.context.address, (error, initialMappingsLocation) => {
           if (error) {
@@ -96,6 +98,8 @@ class StorageViewer {
         })
       })
     }
+    return this.completeMapingsLocationPromise
+  }
 
   /**
     * retrieve mapping location changes from the storage changes.