silence uninformative error after no selection

prevents:

```
Error in if (selection$range$end[["column"]] == 1L) { :
  argument is of length zero
```

The same error is shown in the dialog box.

R/rstudioapi.R

@@ -89,8 +89,7 @@ get_primary_selection <- function(context) {
   selection <- rstudioapi::primary_selection(context)
 
   if (selection[["text"]] == "") {
-    rstudioapi::showDialog("Error", "No code selected. Please highlight some code first.")
-    return(NULL)
+    stop("No code selected. Please highlight some code first.", call. = FALSE)
   }
 
   selection