Fix expression label bug (#5577)

Author: teunbrand

R/guide-axis-theta.R

@@ -185,7 +185,7 @@ GuideAxisTheta <- ggproto(
     key <- vec_slice(key, !vec_detect_missing(key$.label %||% NA))
 
     # Early exit if drawing no labels
-    labels <- key$.label
+    labels <- validate_labels(key$.label)
     if (length(labels) < 1) {
       return(zeroGrob())
     }
@@ -255,7 +255,7 @@ GuideAxisTheta <- ggproto(
 
     key <- params$key
     key <- vec_slice(key, !is.na(key$.label) & nzchar(key$.label))
-    labels <- key$.label
+    labels <- validate_labels(key$.label)
     if (length(labels) == 0 || inherits(elements$text, "element_blank")) {
       return(list(offset = offset))
     }