Refactor hasExercises computed property in VocabLessonView for improved validation

- Enhanced the hasExercises method to include comprehensive checks for lesson and grammarExercises, ensuring robust validation before accessing properties.
- Removed redundant computed property declaration to streamline the code structure.
- Added additional logging for important vocabulary and grammar explanations to aid in debugging and provide better insights during lesson loading.

frontend/src/views/social/VocabLessonView.vue

@@ -264,7 +264,10 @@ export default {
   computed: {
     ...mapGetters(['user']),
     hasExercises() {
-      return this.lesson && this.lesson.grammarExercises && this.lesson.grammarExercises.length > 0;
+      if (!this.lesson) return false;
+      if (!this.lesson.grammarExercises) return false;
+      if (!Array.isArray(this.lesson.grammarExercises)) return false;
+      return this.lesson.grammarExercises.length > 0;
     },
     grammarExplanations() {
       // Extrahiere Grammatik-Erklärungen aus den Übungen
@@ -365,9 +368,6 @@ export default {
       }
     }
   },
-  computed: {
-    ...mapGetters(['user'])
-  },
   watch: {
     courseId() {
       this.loadLesson();
@@ -390,6 +390,8 @@ export default {
         console.log('[VocabLessonView] Anzahl Übungen:', this.lesson?.grammarExercises?.length || 0);
         console.log('[VocabLessonView] activeTab:', this.activeTab);
         console.log('[VocabLessonView] hasExercises:', this.hasExercises);
+        console.log('[VocabLessonView] importantVocab:', this.importantVocab);
+        console.log('[VocabLessonView] grammarExplanations:', this.grammarExplanations);
         // Initialisiere Übungen aus der Lektion oder lade sie separat
         if (this.lesson && this.lesson.id) {
           if (this.lesson.grammarExercises && this.lesson.grammarExercises.length > 0) {