lua - do not crash in the presence of malformed tabset contents

news/changelog-1.8.md

@@ -35,3 +35,7 @@ All changes included in 1.8:
 ## Crossrefs
 
 - ([#12615](https://github.com/quarto-dev/quarto-cli/pull/12615)): Adds `algorithm` to theorem environments. (author: @jeremy9959)
+
+## Lua Filters
+
+- ([#12727](https://github.com/quarto-dev/quarto-cli/issues/12727)): Do not crash in the presence of malformed tabset contents.

src/resources/filters/customnodes/panel-tabset.lua

@@ -146,6 +146,10 @@ _quarto.ast.add_handler({
 
   constructor = function(params)
     local node = _quarto.ast.create_custom_node_scaffold("Tabset", "Block")
+    if params.tabs == nil then
+      warn("No tabs found in tabset. Please check that your markdown includes tab headings as appropriate.")
+      params.tabs = pandoc.List()
+    end
 
     local custom_data = {
       __quarto_custom_node = node,

tests/docs/smoke-all/2025/05/13/issue-12727.qmd

@@ -0,0 +1,35 @@
+---
+title: "Geographic data analysis in R and Python: comparing code and outputs for vector data"
+---
+
+::: {.panel-tabset group="language"}
+
+## Python
+
+```python
+print("ok")
+```
+
+## R
+
+```r
+print("hello")
+```
+
+:::
+
+::: {.panel-tabset group="language"}
+
+## Python
+
+```python
+print("Missing the headers here but we shouldn't crash")
+```
+
+## R
+
+```r
+cat("hello")
+```
+
+:::