How to show triple-state, double-action column via Material icons?

I have this boolean field that can be null too. So I have three states: null, true, false.

Based on it, I have two actions: make true, make false

Is there a good UI/UX way to combine all of them in one understandable UI element?

Right now I have this design. If a field is null, it's showed as false. If field is false, it's showed as a cancel material icon. If it's true, it's showed as a check icon. On hover, if it's false or null, click to approve would be shown, and in case it's true click to disapprove would be shown.:

enter image description here

These are the problems I have with this design:

  • Clickability of items is not apparent. Thous users with my system soon realize it, since I have consistency
  • Null items need to be shown differently, and they can't be shown empty, since empty won't be clickable.

How can I improve this?

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