Error: "NullReferenceException: Object reference not set to an instance of an object" can't solve it

Hi, im a beginner on unity and i'm trying to create a moba type game. To add a fog of war i downloaded the "Ultimate fog of war" asset. After adding the fog of war Manager to my scene and seting it up i can launch the game and it works pretty well, but when i stop the game and launch it again right after, i get these 3 errors at the same time:

NullReferenceException: Object reference not set to an instance of an object
FogOfWarManager.InitializeMaps () (at...

Error: "NullReferenceException: Object reference not set to an instance of an object" can't solve it