warning webdriverexception thrown by findelement selenium

When I run all the tests, some of them crash and I have a notification in the console "WARNING: WebDriverException thrown by findElement(By.xpath: some locator)" and "Session ID is null. Using WebDriver after calling quit()?". I have concluded that the failing tests are those that invoke the methods in which it was used WebDriverWait. But, I'm confused because when I run each test individually, each one passes, even those that failed when I ran them together. Does anyone know how I could solve this problem? Thank you.

Code:

package methods;

import com.relevantcodes.extentreports.ExtentReports;
import com.relevantcodes.extentreports.ExtentTest;
import com.relevantcodes.extentreports.LogStatus;
import org.testng.ITestResult;
import org.testng.annotations.AfterMethod;
 import org.testng.annotations.BeforeMethod;

 import java.io.InputStream;
import java.util.Properties;
 import java.util.concurrent.TimeUnit;

 public class PropertyFile extends SetUp {

 public static Properties properties;
 public static InputStream inputStream;
 public ExtentReports report;
 public ExtentTest test;


@BeforeMethod
public void readPropertiesFile() {

try {

    String workingDir = System.getProperty("user.dir");
    System.out.println(workingDir);

    properties = new Properties();
    inputStream = readFile("application.properties");

    properties.load(inputStream);

    //uzima vrednost do chromeDrivera
    String driverPath = properties.getProperty("DRIVER_PATH_WIN_CHROME");
    //uzima vrednost do report-a
    String reportPath = properties.getProperty("FOLDER_REPORT_PATH");


    System.setProperty(properties.getProperty("DRIVER_NAME_CHROME"), 
    workingDir + 
    driverPath);
    System.out.println(workingDir + driverPath);

    // kreiranje reporta
    report = new ExtentReports(workingDir + reportPath, false);
    //URL do sajta koji testiramo
    report.addSystemInfo("URL", "https://demoqa.com/");

    createDriver();

    getDriver().manage().timeouts().implicitlyWait(15, TimeUnit.SECONDS);

    // govorimo pretrazivacu koju stranicu da otvori
    getDriver().get(properties.getProperty("URL"));

    } catch (Exception e) {
    System.out.println("Failed to instantiate and load properties and chrome 
     driver!");
    }
   }
 @Test
 public void check(){
    test = report.startTest("exampe");

    slider.moveSliderTo(100);
    test.log(LogStatus.INFO, "Slider was moved");

    MainMethods.waitElementToBeVisible(By.xpath("//span[@style = 'left: 
 20%;']"));
    test.log(LogStatus.INFO, "Position of slider is on 20%");
}

@AfterMethod
public void tearDown(ITestResult testResult) throws Exception {

if (testResult.getStatus() == ITestResult.FAILURE) {
    String path = methods.Screenshot.takeScreenshot(getDriver(), 
methods.Screenshot.generateFileName(testResult));
    String imgPath = test.addScreenCapture(path);
    test.log(LogStatus.FAIL, "Test case which FAILED is " + 
testResult.getName(), 
imgPath);
    test.log(LogStatus.FAIL, "Method which FAILED is " + 
testResult.getThrowable());
} else if (testResult.getStatus() == ITestResult.SUCCESS) {
    test.log(LogStatus.PASS, "Test case which PASSED IS " + 
testResult.getName());
} else if (testResult.getStatus() == ITestResult.SKIP) {
    test.log(LogStatus.PASS, "Test case which SKIPPED IS " + 
 testResult.getName());
}

getDriver().manage().timeouts().implicitlyWait(30, TimeUnit.SECONDS);
quitDriver();
report.endTest(test);
report.flush();
}