Don’t err in pip-frame-remove-buffer if there is no PIP frame

This is useful e.g. to prevent the error when a PIP frame with a
temporarily displayed buffer is deleted before the buffer timer
attempts to remove the buffer.

README.org

@@ -43,6 +43,26 @@ this utility:
 
 * How?
 
+** Installation
+
+You can install pip-frame.el from [[https://melpa.org/#/pip-frame][MELPA]].  If you use [[https://github.com/radian-software/straight.el][straight.el]] and
+prefer using source repos directly, you can install pip-frame.el as
+follows:
+
+   #+begin_src elisp
+(straight-use-package
+ '(pip-frame :type git
+             :repo "https://git.zamazal.org/pdm/pip-frame"
+             :local-repo "pip-frame"))
+   #+end_src
+
+For manual installation, simply put [[./pip-frame.el][pip-frame.el]] file to your
+=site-lisp= directory and add the following to your Emacs configuration:
+
+   #+begin_src elisp
+(require ’pip-frame)
+   #+end_src
+
 ** Commands
 
 The PIP frame is created using =M-x pip-frame-add-buffer= command.  You

pip-frame.el

@@ -33,10 +33,10 @@
 ;; `M-x pip-frame-remove-buffer' or close the whole frame with
 ;; `M-x pip-frame-delete-frame'.
 
-(require 'cl-lib)
-
 ;;; Code:
 
+(require 'cl-lib)
+
 (defgroup pip-frame ()
   "Display PIP frame."
   :group 'frames)
@@ -175,7 +175,8 @@ If the buffer is not present in the PIP frame, do nothing."
   (interactive (list (completing-read "Remove PIP buffer: "
                                       (mapcar #'buffer-name (pip-frame--buffers))
                                       nil t)))
-  (let* ((windows (window-list (pip-frame--get-frame)))
+  (if-let ((frame (pip-frame--get-frame t))
+           (windows (window-list frame))
            (buffer (get-buffer buffer-or-name))
            (windows-to-delete (cl-remove buffer windows
                                          :key #'window-buffer
@@ -233,4 +234,8 @@ Any other key stops this command and executes its own command."
 
 (provide 'pip-frame)
 
+;; Local Variables:
+;; checkdoc-force-docstrings-flag: nil
+;; End:
+
 ;;; pip-frame.el ends here