Added try-catch to notification close in symbol destructor

[RobertoRoos]

Fixes #303

[coveralls]

Pull Request Test Coverage Report for Build 1717174302

• 3 of 5 (60.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 94.432%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pyads/symbol.py	3	5	60.0%
<!--	Total:	3	5	60.0%	-->

Totals
Change from base Build 1576796242:	-0.1%
Covered Lines:	1696
Relevant Lines:	1796

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💛 - Coveralls

[stlehmann]

@RobertoRoos do you think you can find a way to add a test for this particular case?