Preparation of 1-​(5-​tert-​butyl-​2-​aryl-​pyrazol-​3-​yl)​-​3-​[2-​fluoro-​4-​[(3-​oxo-​4H-​pyrido[2,​3-​b]​pyrazin-​8-​yl)​oxy]​phenyl]​urea derivatives as RAF inhibitors for treating cancer