chrome.tabCapture.capture返回null流

Question

我遇到这段代码的问题，似乎传递给chroma.tabCapture.capture的回调的流是空的

var receiver = null;
chrome.tabs.getSelected(null,function(tab) {
   chrome.tabCapture.capture({
      video: true,
      audio: false,
      videoConstraints: {
        mandatory: {
          minWidth: 16,
          minHeight: 9,
          maxWidth: 1280,
          maxHeight: 720,
          maxFrameRate: 25,  // Note: Frame rate is variable (0 <= x <= 60).
        },
      },
    },
    function(stream) {
      alert(stream);
      if (!stream) {
        console.log('Error starting tab capture: ' + (chrome.runtime.lastError.message || 'UNKNOWN'));
        return;
      }
      if (receiver != null) {
        receiver.close();
      }
      receiver = window.open('receiver.html');
      receiver.currentStream = stream;
    }
  );
});

警报（流）;为null，因此代码在打开新选项卡之前返回，并传递流。非常感谢帮助