fix(undefined issue): add type guard for data

src/index.ts

@@ -45,7 +45,7 @@ interface Level {
  */
 interface ConstructorArgs {
   /** Previously saved data */
-  data: HeaderData;
+  data: HeaderData | {};
   /** User config for the tool */
   config: HeaderConfig;
   /** Editor.js API */
@@ -116,7 +116,7 @@ export default class Header {
      * @type {HeaderData}
      * @private
      */
-    this._data = this.normalizeData(data);
+    this._data = this.normalizeData(data as HeaderData);
 
     /**
      * Main Block wrapper
@@ -136,6 +136,17 @@ export default class Header {
     };
   }
 
+  /**
+   * Check if data is valid
+   * 
+   * @param {any} data - data to check
+   * @returns {data is HeaderData}
+   * @private
+   */
+  isHeaderData(data: any): data is HeaderData {
+    return (data as HeaderData).text !== undefined && (data as HeaderData).level !== undefined;
+  }
+
   /**
    * Normalize input data
    *
@@ -147,12 +158,12 @@ export default class Header {
   normalizeData(data: HeaderData): HeaderData {
     const newData: HeaderData = { text: '', level: this.defaultLevel.number };
 
-    if (typeof data !== 'object') {
+    if (!this.isHeaderData(data)) {
       data = { text: '', level: this.defaultLevel.number};
     }
 
     newData.text = data.text || '';
-    newData.level = data.level ? parseInt(data.level.toString()) : this.defaultLevel.number;
+    newData.level = parseInt(data.level.toString()) || this.defaultLevel.number;
 
     return newData;
   }
@@ -275,7 +286,7 @@ export default class Header {
   get data(): HeaderData {
     this._data.text = this._element.innerHTML;
     this._data.level = this.currentLevel.number;
-
+    
     return this._data;
   }