Change member 'fileCannotOpenDialog' in CopyFileActivity to a local variable

src/main/java/com/kdanmobile/reader/copyfile/CopyFileActivity.kt

@@ -21,8 +21,6 @@ abstract class CopyFileActivity : KdanBaseActivity() {
         const val KEY_SHOULD_COPY_FILE = "should_copy_file"
     }
 
-    private var fileCannotOpenDialog: FileCannotOpenDialogFragment? = null
-
     private val viewModel: CopyFileViewModel by viewModel {
         val kdanPdfReaderFolder = getKdanPdfReaderFolder()
         FileUtil.init(kdanPdfReaderFolder)
@@ -60,11 +58,11 @@ abstract class CopyFileActivity : KdanBaseActivity() {
     }
 
     private fun showFileCannotOpenDialog(title: String) {
-        fileCannotOpenDialog = FileCannotOpenDialogFragment()
-        fileCannotOpenDialog?.arguments = Bundle().also {
+        val fileCannotOpenDialog = FileCannotOpenDialogFragment()
+        fileCannotOpenDialog.arguments = Bundle().also {
             it.putString(FileCannotOpenDialogFragment.KEY_DIALOG_TITLE, title)
         }
-        fileCannotOpenDialog?.show(supportFragmentManager, fileCannotOpenDialog?.tag)
+        fileCannotOpenDialog.show(supportFragmentManager, fileCannotOpenDialog.tag)
     }
 
     private fun startReaderActivity(filePath: String?) {