Selenium: Unable to click a button of custom popup - selenium

Question

I tried to click a button of select files then submit button in the popup but it neither recognizes the popup nor the button in it.
For selecting the modal-content
driver.switchTo().frame("modal-content");
message:
Exception in thread "main" org.openqa.selenium.NoSuchFrameException: No frame element found by name or id modal-content
Build info: version: '3.141.59', revision: 'e82be7d358', time: '2018-11-14T08:25:48'
System info: host: 'RAUNAK-MA', ip: '192.168.2.200', os.name: 'Windows 10', os.arch: 'amd64', os.version: '10.0', java.version: '1.8.0_201'
Driver info: driver.version: unknown
at org.openqa.selenium.remote.RemoteWebDriver$RemoteTargetLocator.frame(RemoteWebDriver.java:885)
at newpackage1.newTest.main(newTest.java:41)
post removing the switch to and directly selecting the element
WebElement LinkCheckerbutton1 = driver.findElement(By.id("linkCheckerFileUpload"));
LinkCheckerbutton1.click();
Results:
Exception in thread "main" org.openqa.selenium.NoSuchElementException: no such element: Unable to locate element: {"method":"id","selector":"linkCheckerFileUpload"}
(Session info: chrome=73.0.3683.103)
(Driver info: chromedriver=2.46.628402 (536cd7adbad73a3783fdc2cab92ab2ba7ec361e1),platform=Windows NT 10.0.17134 x86_64) (WARNING: The server did not provide any stacktrace information)
Command duration or timeout: 0 milliseconds
For documentation on this error, please visit: https://www.seleniumhq.org/exceptions/no_such_element.html
Build info: version: '3.141.59', revision: 'e82be7d358', time: '2018-11-14T08:25:48'
System info: host: 'RAUNAK-MA', ip: '192.168.2.200', os.name: 'Windows 10', os.arch: 'amd64', os.version: '10.0', java.version: '1.8.0_201'
Driver info: org.openqa.selenium.chrome.ChromeDriver
Capabilities {acceptInsecureCerts: false, acceptSslCerts: false, applicationCacheEnabled: false, browserConnectionEnabled: false, browserName: chrome, chrome: {chromedriverVersion: 2.46.628402 (536cd7adbad73a..., userDataDir: C:\Users\RAUNAK~1.MAS\AppDa...}, cssSelectorsEnabled: true, databaseEnabled: false, goog:chromeOptions: {debuggerAddress: localhost:65266}, handlesAlerts: true, hasTouchScreen: false, javascriptEnabled: true, locationContextEnabled: true, mobileEmulationEnabled: false, nativeEvents: true, networkConnectionEnabled: false, pageLoadStrategy: normal, platform: XP, platformName: XP, proxy: Proxy(), rotatable: false, setWindowRect: true, strictFileInteractability: false, takesHeapSnapshot: true, takesScreenshot: true, timeouts: {implicit: 0, pageLoad: 300000, script: 30000}, unexpectedAlertBehaviour: ignore, unhandledPromptBehavior: ignore, version: 73.0.3683.103, webStorageEnabled: true}
Session ID: 889a04bb870854aef890e9dcb55c7508
*** Element info: {Using=id, value=linkCheckerFileUpload}
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
at java.lang.reflect.Constructor.newInstance(Unknown Source)
at org.openqa.selenium.remote.ErrorHandler.createThrowable(ErrorHandler.java:214)
at org.openqa.selenium.remote.ErrorHandler.throwIfResponseFailed(ErrorHandler.java:166)
at org.openqa.selenium.remote.http.JsonHttpResponseCodec.reconstructValue(JsonHttpResponseCodec.java:40)
at org.openqa.selenium.remote.http.AbstractHttpResponseCodec.decode(AbstractHttpResponseCodec.java:80)
at org.openqa.selenium.remote.http.AbstractHttpResponseCodec.decode(AbstractHttpResponseCodec.java:44)
at org.openqa.selenium.remote.HttpCommandExecutor.execute(HttpCommandExecutor.java:158)
at org.openqa.selenium.remote.service.DriverCommandExecutor.execute(DriverCommandExecutor.java:83)
at org.openqa.selenium.remote.RemoteWebDriver.execute(RemoteWebDriver.java:552)
at org.openqa.selenium.remote.RemoteWebDriver.findElement(RemoteWebDriver.java:323)
at org.openqa.selenium.remote.RemoteWebDriver.findElementById(RemoteWebDriver.java:372)
at org.openqa.selenium.By$ById.findElement(By.java:188)
at org.openqa.selenium.remote.RemoteWebDriver.findElement(RemoteWebDriver.java:315)
at newpackage1.newTest.main(newTest.java:43)

Answer 1

This error message...
Exception in thread "main" org.openqa.selenium.NoSuchFrameException: No frame element found by name or id modal-content
...implies that there is no frame element by the id or name modal-content.
However it gives us a hint the desired element is within a Modal Dialog Box.
So to locate the element you have to induce WebDriverWait for the elementToBeClickable() and you can use either of the following Locator Strategies:
cssSelector:
new WebDriverWait(driver, 20).until(ExpectedConditions.elementToBeClickable(By.cssSelector("#linkCheckerFileUpload"))).click();
xpath:
new WebDriverWait(driver, 20).until(ExpectedConditions.elementToBeClickable(By.xpath("//*[id='linkCheckerFileUpload']"))).click();