fix: yaml worker errors at undefined row/column

Author: akoreman

lib/ace/mode/yaml_worker.js

@@ -34,9 +34,11 @@ oop.inherits(YamlWorker, Mirror);
                 return;
             }
 
+            // error.mark is not always defined, if undefined we default to displaying the error at the top row of the document.
+            var markDefined = !!error.mark;
             errors.push({
-                row: error.mark.line,
-                column: error.mark.column,
+                row: markDefined ? error.mark.line : 0,
+                column: markDefined ? error.mark.column : 0,
                 text: error.reason,
                 type: 'error',
                 raw: error