Crash when creating a WebGL build without switching the platform.

Steps to repro:
1. Create a new project.
2. Select WebGL platform from the build settings. (Current state of the platform should be other than WebGL in build settings).
3. Build the project for WebGL without switching the platform.

NOTE: The Editor crashes as soon as it starts building. This does not occur if the platform is switched before building.

Reproducible on:
2018.2.0b1, 2018.2.0a9, 2018.2.0a7.

Working fine on:
2018.2.0a6, 2018.1.0f1.

Environment:
Occurring only on Mac.