Provide a better error when trying to insert null elements

[Artur-]

This change is

[pleku]

Reviewed 2 of 2 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[Legioth]

Reviewed 1 of 2 files at r1, 1 of 1 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[Legioth]

Reviewed 1 of 1 files at r3.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[vaadin-tc]

SonarQube analysis reported 1 issue:

• 1 major

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Note: the following issues could not be reported as comments because they are located on lines that are not displayed in this pull request:

• The Cyclomatic Complexity of this method "insertChild" is 11 which is greater than 10 authorized.