[java] Add null check before returning isDisplayed value

[pujagani]

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Description

Add null check before returning isDisplayed value.
Fixes #9266 from Java client end.

Motivation and Context

Related to the issue mentioned above when isDisplayed() throws an NullPointerException randomly when running with the Firefox driver. To get the result of isDisplayed() function the script https://github.com/SeleniumHQ/selenium/blob/trunk/rb/lib/selenium/webdriver/atoms/isDisplayed.js is run on the driver end. Sometimes the Firefox driver returns null and a null check would safeguard against that. It could be similar to #4555.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[diemol]

Thank you, @pujagani!

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

0.0% Coverage
0.0% Duplication

[PiotrZgadzaj]

I don't know what is selenium release calendar, but do you plan to include that in some selenium 3.x release? If so then when ?

[diemol]

We do not backport fixes to previous releases, so this will be available in beta-3, which should be released (I guess) by mid or end of April.