fixes #669 by updating main_window.py

[chauhan-shobhit]

What are the issues this pull addresses (issue numbers / links)?

Issue #669 https://github.com/Qiskit/qiskit-metal/issues/669

Did you add tests to cover your changes (yes/no)?

no

Did you update the documentation accordingly (yes/no)?

yes

Did you read the CONTRIBUTING document (yes/no)?

yes

Summary

Fixed the issue #669 by updating save_design function in qiskit_metal/_gui/main_window.py

Details and comments

When using cancel instead of save, QFileDialog.getSaveFileName returns empty string( https://github.com/Qiskit/qiskit-metal/blob/main/qiskit_metal/_gui/main_window.py#L163 ) as filename. This caused a FileNotFoundError during opening of file (https://github.com/Qiskit/qiskit-metal/blob/main/qiskit_metal/_gui/main_window.py#L171).

To fix the issue , an empty check was added. File will be opened and written to only if filename is non-empty (length of string is non-zero)

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