Fix "Trying to access array offset on null" warning

framework/caching/FileCache.php

@@ -158,8 +158,14 @@
             return @touch($cacheFile, $duration + time());
         }
 
-        $error = error_get_last();
-        Yii::warning("Unable to write cache file '{$cacheFile}': {$error['message']}", __METHOD__);
+        $message = "Unable to write cache file '{$cacheFile}'";
+
+        if ($error = error_get_last()) {
+            $message .= ": {$error['message']}";
+        }
+
+        Yii::warning($message, __METHOD__);
+
         return false;
     }