java.lang.NoClassDefFoundError was not included in the set of types which can be serialized

I get an error when launching a GWT app, gwt 2.8.1 / jdk 8

java.lang.NoClassDefFoundError was not included in the set of types which can be serialized by this Seriaiazation policy or its Class object could not be loaded. For security purposes, this type will not be serialized. : instance = java.lang.NoClassDefFoundError: com/google/gwt/core/client/GWTBridge

(please check the image below for full logs)

First I can't make this class serializable as seen on others posts.

Second I don't have the GWTBridge in that package, it is inside com.google.gwt.core.shared

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