fix #1442: make sure the fallback index file list is of length 1 before checking if it exists

Author: yihui

DESCRIPTION

@@ -1,7 +1,7 @@
 Package: bookdown
 Type: Package
 Title: Authoring Books and Technical Documents with R Markdown
-Version: 0.35.1
+Version: 0.35.2
 Authors@R: c(
     person("Yihui", "Xie", role = c("aut", "cre"), email = "[email protected]", comment = c(ORCID = "0000-0003-0645-5666")),
     person("Christophe", "Dervieux", , "[email protected]", role = c("ctb"),

NEWS.md

@@ -2,6 +2,7 @@
 
 - Fix an issue with parsing resources from raw HTML code (thanks, @lennylin, https://community.rstudio.com/t/bookdown-image-with-a-weblink/172542)
 
+- R 4.3.x would error if multiple files are passed to `render_book()` without an `output_format` specified (thanks, @slodge-work, #1442).
 
 # CHANGES IN bookdown VERSION 0.35
 

R/utils.R

@@ -413,7 +413,7 @@ get_output_formats = function(fallback_format, filter = identity, first = FALSE,
   index = get_index_file()
   # Use fallback file unless no YAML
   if (is_empty(index)) {
-    if (!is.null(fallback_index) &&
+    if (length(fallback_index) == 1 &&
         xfun::file_exists(fallback_index) &&
         length(rmarkdown::yaml_front_matter(fallback_index)) != 0
     ) {