退出document.evaluate

try/catch

if (typeof xpathExpression !== "undefined" && xpathExpression !== "") {
    document.evaluate(
        xpathExpression,
        document,
        null,
        XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE,
        null);
}

try-catch

try {
  document.evaluate(...);
} catch (ex) {
  // something went wrong
}

if (xpathExpression)
document.evaluate(...);