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All that will really happen is that the Pokemon will be unable to evolve.
The games will not allow an evolution to occur if the thing you're evolving into isn't in the Kanto Dex and you don't have the National Dex - it'll just mysteriously "stop evolving". It's actually in place to prevent, for example, Golbat evolving into Crobat, but it affects all Pokemon who have such evolutions.
No, it will not crash your game. I tried to trade a Scyther holding a metal coat to my Leaf Green game before I had gotten the National Dex, and when it tried to evolve, I got the "Huh? SCYTHER stopped evolving!" message because Scizor was introduced in Gen 2, and is not in the Kanto dex.