Fix sometimes player cannot be determined and yield nil unwrap (#2231)

Fix the issue metioned in #2207

iina/PlaylistViewController.swift

@@ -115,6 +115,7 @@ class PlaylistViewController: NSViewController, NSTableViewDataSource, NSTableVi
    // register for drag and drop
    playlistTableView.registerForDraggedTypes([.iinaPlaylistItem, .nsFilenames, .nsURL, .string])

    (subPopover.contentViewController as! SubPopoverViewController).player = player
    if let popoverView = subPopover.contentViewController?.view,
      popoverView.trackingAreas.isEmpty {
      popoverView.addTrackingArea(NSTrackingArea(rect: popoverView.bounds,
@@ -770,10 +771,7 @@ class SubPopoverViewController: NSViewController, NSTableViewDelegate, NSTableVi
  @IBOutlet weak var playlistTableView: NSTableView!
  @IBOutlet weak var heightConstraint: NSLayoutConstraint!

  lazy var playerCore: PlayerCore = {
    let windowController = self.playlistTableView.window!.windowController
    return (windowController as? MainWindowController)?.player ?? (windowController as! MiniPlayerWindowController).player
  }()
  weak var player: PlayerCore!

  var filePath: String = ""

@@ -782,18 +780,18 @@ class SubPopoverViewController: NSViewController, NSTableViewDelegate, NSTableVi
  }

  func tableView(_ tableView: NSTableView, objectValueFor tableColumn: NSTableColumn?, row: Int) -> Any? {
    guard let matchedSubs = playerCore.info.matchedSubs[filePath] else { return nil }
    guard let matchedSubs = player.info.matchedSubs[filePath] else { return nil }
    return matchedSubs[row].lastPathComponent
  }

  func numberOfRows(in tableView: NSTableView) -> Int {
    return playerCore.info.matchedSubs[filePath]?.count ?? 0
    return player.info.matchedSubs[filePath]?.count ?? 0
  }

  @IBAction func wrongSubBtnAction(_ sender: AnyObject) {
    playerCore.info.matchedSubs[filePath]?.removeAll()
    player.info.matchedSubs[filePath]?.removeAll()
    tableView.reloadData()
    if let row = playerCore.info.playlist.index(where: { $0.filename == filePath }) {
    if let row = player.info.playlist.index(where: { $0.filename == filePath }) {
      playlistTableView.reloadData(forRowIndexes: IndexSet(integer: row), columnIndexes: IndexSet(integersIn: 0...1))
    }
  }