问使用WebDriverWait布尔值的NullPointerException

EN

public void methodThatRuns(WebDriver driver) throws InterruptedException {
    properties.inputTxt(driver, "100");
    sundries.waitEnabledButton(driver, properties.nextButton(driver));
    properties.nextButton(driver).click();
}   

public void waitEnabledButton(WebDriver driver, final WebElement btn) throws NullPointerException {
    WebDriverWait wait = new WebDriverWait(driver, 10);
    System.out.println("Starting the wait");
    try {
        wait.until(new ExpectedCondition<Boolean>(){
            public Boolean apply(WebDriver driver) {
                final String attribute = btn.getAttribute("disabled");
                if (attribute.equals(null)) {
                    return true;
                }
                else {
                    return false;
                }
            }
        });
    } catch (StaleElementReferenceException e) {
        System.out.println("The disabled attribute was destroyed successfully and the script can continue."); //using this as the attribute gets destroyed when the button is enabled which throws a staleElement exception
    } 
    System.out.println("Wait is over");
}