The logs
```
  Stack:
    Error: Protocol error (Target.createTarget): can't access property "closing", selectedTab is null _setPositionalAttributes@chrome://browser/content/tabbrowser-tabs.js:1123:11
    _insertTabAtIndex@chrome://browser/content/tabbrowser.js:3286:25
    addTab@chrome://browser/content/tabbrowser.js:2710:16
    addTab@chrome://remote/content/shared/TabManager.jsm:24:31
    createTarget@chrome://remote/content/cdp/domains/parent/Target.jsm:90:28
    execute@chrome://remote/content/cdp/domains/DomainCache.jsm:101:25
    execute@chrome://remote/content/cdp/sessions/Session.jsm:64:25
    onPacket@chrome://remote/content/cdp/CDPConnection.jsm:248:36
    onMessage@chrome://remote/content/server/WebSocketTransport.jsm:89:18
    handleEvent@chrome://remote/content/server/WebSocketTransport.jsm:71:14
    openPromptSync@resource://gre/modules/Prompter.jsm:1057:17
    alert@resource://gre/modules/Prompter.jsm:1329:17
    alert@resource://gre/modules/Prompter.jsm:80:7
    _displayPrintingError@chrome://global/content/printUtils.js:572:21
```
My feeling is that something is wrong with the printing test and an alert is generated which blocks following attempts to get the current selected tab (see https://searchfox.org/mozilla-central/source/toolkit/components/printing/content/printUtils.js#572).
});
  describe("in doc_actions.pdf for page actions", () => {
    let pages;
    beforeAll(async () => {
      pages = await loadAndWait("doc_actions.pdf", "#\\34 7R");
    });
    afterAll(async () => {
      await closePages(pages);
    });
    it("must execute PageOpen and PageClose actions", async () => {
      await Promise.all(
        pages.map(async ([browserName, page]) => {
          await page.waitForFunction(
            "window.PDFViewerApplication.scriptingReady === true"
          );
          let text = await page.$eval("#\\34 7R", el => el.value);
          expect(text).withContext(`In ${browserName}`).toEqual("PageOpen 1");
          for (let run = 0; run < 5; run++) {
            for (const ref of [18, 19, 20, 21, 47, 50]) {
              await page.evaluate(refElem => {
                const element = window.document.getElementById(`${refElem}R`);
                if (element) {
                  element.value = "";
                }
              }, ref);
            }
            for (const [refOpen, refClose, pageNumOpen, pageNumClose] of [
              [18, 50, 2, 1],
              [21, 19, 3, 2],
              [47, 20, 1, 3],