Prevent error on trying to access epubs without progress

reader/views.py

@@ -158,10 +158,12 @@ def read_book(request, book_id):
         case "epub":
             reader_template = "reader/book_read/epub_viewer.html"
             # Try to parse CFI
+            if userbook.last_progress_cfi is not None:
                 rgx = re.compile(r"epubcfi\((.*)\)")
                 m = rgx.match(userbook.last_progress_cfi)
                 if m:
                     cfi_ref = userbook.last_progress_cfi
+
         case _:
             reader_template = "reader/book_read/invalid_filetype.html"
     return render(request, reader_template, {