fixed a bug

android/sample-app/src/main/java/cn/learningpad/oraltrainer/sample/CourseCatalogController.kt

@@ -128,7 +128,7 @@ internal class CourseCatalogController(
                     return
                 }
                 host.onStatus("句子边界加载失败：${error.message.orEmpty()}")
-                host.onSentencePlaceholder("暂无句子边界", video.title)
+                host.onSentencePlaceholder("暂无句子边界", "")
             }
         })
     }

android/sample-app/src/main/java/cn/learningpad/oraltrainer/sample/MainActivity.kt

@@ -214,7 +214,7 @@ class MainActivity : Activity() {
                     bindings.statusText.text =
                         "句子边界获取失败，使用默认快退/快进：${error.message.orEmpty()}"
                     bindings.sentenceMetaText.text = "暂无句子边界"
-                    bindings.sentenceText.text = item.title
+                    bindings.sentenceText.text = ""
                 }
             }
         })
@@ -372,7 +372,7 @@ class MainActivity : Activity() {
         if (!bindings.hasPlayerChrome) return
         if (sentence == null) {
             bindings.sentenceMetaText.text = "暂无句子边界"
-            bindings.sentenceText.text = bindings.lessonTitleText.text
+            bindings.sentenceText.text = ""
             bindings.explainSentenceButton?.isEnabled = false
             return
         }