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This patch fixes a TypeScript error when using the `Replay` integration exported from the NextJS SDK (>= 7.27.0). Because our SDK tells TS that it should import types from `index.server.ts`, TS doesn't find the Replay integration class although it's correctly exported from `indext.client.ts`. 

Note: even without this change, Replay works in NextJS, it just fixes just a TS error.

Setting an `Event`'s property `type: undefined`, like we do in the SDK for `ErrorEvent`s causes a TS error, whenever the recommended `exactOptionalPropertyTypes` TS option is enabled. This option was introduced in TS 4.4. Since we're on TS 3.8, we couldn't catch this error. 

This patch changes to `EventType` type declaration to also accept `undefined` as a value, thereby resolving the TS error.