do not call correctPNG() when undefined

prefs.js

@@ -42,7 +42,9 @@ function feedlist_callback() {
 
 				caller_subop = false;
 			}
-			correctPNG();
+			if (typeof correctPNG != 'undefined') {
+				correctPNG();
+			}
 			notify("");
 		} catch (e) {
 			exception_error("feedlist_callback", e);
@@ -54,7 +56,9 @@ function filterlist_callback() {
 	var container = document.getElementById('prefContent');
 	if (xmlhttp.readyState == 4) {
 		container.innerHTML=xmlhttp.responseText;
-		correctPNG();
+		if (typeof correctPNG != 'undefined') {
+			correctPNG();
+		}
 		notify("");
 	}
 }
@@ -77,7 +81,9 @@ function labellist_callback() {
 				checkbox.checked = true;
 			}
 		}
-		correctPNG();
+		if (typeof correctPNG != 'undefined') {
+			correctPNG();
+		}
 		notify("");
 	}
 }