Update ShaderGenerator.js

Author: LalitNarayanYadav

src/webgl/ShaderGenerator.js

@@ -11,6 +11,7 @@ import escodegen from 'escodegen';
 function shadergenerator(p5, fn) {
    if (!fn || typeof fn !== 'object') {
     throw new Error("fn is not defined or is not an object. Ensure it is passed correctly to shadergenerator.");
+   }
 
   let GLOBAL_SHADER;
   let BRANCH;