Check if the dispose has already been done and does nothing | Fixes #582

[matuzalemsteles]

hey @carloslancha, the messages from warn happened when the dispose was made on the components that used ClayAlertBase, the dispose happened from top to bottom and when it arrived in ClayAlertBase in some cases it had no more reference of _visible... The way to omit this is to check when it has already been disposed.

[coveralls]

Pull Request Test Coverage Report for Build 335

• 6 of 7 (85.71%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.006%) to 82.148%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/clay-alert/src/ClayAlertBase.js	6	7	85.71%

Totals
Change from base Build 328:	0.006%
Covered Lines:	4459
Relevant Lines:	4698

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💛 - Coveralls

[carloslancha]

Makes sense, thx @matuzalemsteles