Type 'null' is not assignable to type 'T'

You have to declare the return type as null or turn off strictNullChecks in your tsconfig

public static bar<T>(x: T): T | null

or you could type null as any e.g.

 return null as any;

Since version 3.9.5, TypeScript enforces strictNullChecks on numbers and strings just to name a few. For example, the following code will throw an error during compilation:

let x: number = null;

To avoid this error you have two options:

• Set strictNullChecks=false in tsconfig.json.
• Declare your variable type as any:

  let x: any = null;