我有以下代码：

private var bookmarkDetailsView: LiveData<BookmarkDetailsView>? = null

private fun mapBookmarkToBookmarkView(bookmarkId: Long) {
    val bookmark = bookmarkRepo.getLiveBookmark(bookmarkId)
    bookmarkDetailsView = Transformations.map(bookmark) { repoBookmark ->
    repoBookmark?.let { repoBookmark ->
          bookmarkToBookmarkView(repoBookmark)
    }
  }
}

因为Transformation是deprecated，所以我使用Map

private fun mapBookmarkToBookmarkView(bookmarkId: Long) {
    val bookmark = bookmarkRepo.getLiveBookmark(bookmarkId)
     bookmarkDetailsView = bookmark.map { repoBookmark ->
         bookmarkToBookmarkView(repoBookmark)
    }
}

然而，一旦书签从数据库中删除，我就会出现以下崩溃：

E FATAL EXCEPTION: main 
Process: com.raywenderlich.placebook, PID: 6512 
java.lang.NullPointerException: Parameter specified as non-null is null: method com.raywenderlich.placebook.viewmodel.BookmarkDetailsViewModel$mapBookmarkToBookmarkView$1.invoke, parameter kausi_repoBookmark at com.raywenderlich.placebook.viewmodel.BookmarkDetailsViewModel$mapBookmarkToBookmarkView$1.invoke(Unknown Source:2)
at com.raywenderlich.placebook.viewmodel.BookmarkDetailsViewModel$mapBookmarkToBookmarkView$1.invoke(BookmarkDetailsViewModel.kt:50)
at androidx.lifecycle.Transformations$map$1.invoke(Transformations.kt:52) 
at androidx.lifecycle.Transformations$map$1.invoke(Transformations.kt:52)  at androidx.lifecycle.Transformations$sam$androidx_lifecycle_Observer$0.onChanged(Unknown Source:2)
at androidx.lifecycle.MediatorLiveData$Source.onChanged(MediatorLiveData.java:171)
at androidx.lifecycle.LiveData.considerNotify(LiveData.java:133)  
at androidx.lifecycle.LiveData.dispatchingValue(LiveData.java:151)  
at androidx.lifecycle.LiveData.setValue(LiveData.java:309)  
at androidx.lifecycle.LiveData$1.run(LiveData.java:93)  
at android.os.Handler.handleCallback(Handler.java:958)  
at android.os.Handler.dispatchMessage(Handler.java:99)  
at android.os.Looper.loopOnce(Looper.java:205)  
at android.os.Looper.loop(Looper.java:294)  
at android.app.ActivityThread.main(ActivityThread.java:8128)  
at java.lang.reflect.Method.invoke(Native Method)  
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:578)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:946

我做错了什么？如果你能解释一下就好了。

更新

fun getLiveBookmark(bookmarkId: Long): LiveData<Bookmark> = bookmarkDao.loadLiveBookmark(bookmarkId)

@Query("SELECT * FROM Bookmark WHERE id =:bookmarkId")
fun loadLiveBookmark(bookmarkId: Long): LiveData<Bookmark>

更新2

我做了以下更改：

fun getLiveBookmark(bookmarkId: Long): LiveData<Bookmark?> = bookmarkDao.loadLiveBookmark(bookmarkId)

现在我得到了另一个错误：

private fun mapBookmarkToBookmarkView(bookmarkId: Long) {
    val bookmark = bookmarkRepo.getLiveBookmark(bookmarkId)
    if (bookmark != null) {
        bookmarkDetailsView = bookmark.map { repoBookmark ->
            repoBookmark?.let { // error is on this line
                bookmarkToBookmarkView(it)
            }
       }
    }
}

错误：

Type mismatch: inferred type is BookmarkDetailsViewModel.BookmarkDetailsView? but BookmarkDetailsViewModel.BookmarkDetailsView was expected