Fixed a TypeError, trying to find 'error' in a None

mjtest/test/tests.py

@@ -281,7 +281,7 @@ class BasicTestResult(TestResult):
                  incorrect_msg: str = "incorrect return code"):
         super().__init__(test_case, error_code)
         self._incorrect_msg = incorrect_msg
-        self._contains_error_str = "error" in error_output
+        self._contains_error_str = error_output is not None and "error" in error_output
         self.error_output = error_output
         self.output = output
         self.other_texts = [] # type: List[Tuple[str, str, bool]]
@@ -420,4 +420,4 @@ class LexerDiffTest(DiffTest):
 TestCase.TEST_CASE_CLASSES[TestMode.lexer].append(LexerDiffTest)
 
 import mjtest.test.syntax_tests
-import mjtest.test.ast_tests
\ No newline at end of file
+import mjtest.test.ast_tests