Fix error in dap--buffers-w-breakpoints when a debug session ends

The error `(wrong-type-argument markerp nil)` occurs when a debug session
finishes and a buffer containing breakpoints was killed beforehand. The fix is
to add a `nil`-check.

Author: josephtesfaye

dap-mode.el

@@ -867,8 +867,9 @@ will be reversed."
   "Get only the buffers featuring at least one breakpoint"
   ;; extract the list of buffers featuring a breakpoint from their first breakpoint marker
   ;; (as stored in the LSP metadata)
-  (--map (marker-buffer (plist-get (car it) :marker))
-         (ht-values (dap--get-breakpoints))))
+  (delq nil (--map (when-let ((marker (plist-get (car it) :marker)))
+                     (marker-buffer marker))
+                   (ht-values (dap--get-breakpoints)))))
 
 (defun dap--refresh-breakpoints ()
   "Refresh breakpoints for DEBUG-SESSION."