QGaphicsView size is not getting set.

I'm currently working on a project in which I have a QgraphicsView inside a tab. And I have one other tab for a purpose.
When I'm switching from one tab to another and then trying to set the size of the view ,it is not getting set.

But when i'm setting the size working on same tab , the code is working well.

Could one please guide me.

Also, please tell whether we can set the size of the QGraphicsView when it's parent class is hidden??

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Added after 47 minutes:

If one is not clear with the question, let me know