Cannot upload custom .ico files for client branding in AhsayCBS

When customizing the backup clients installer via the AhsayCBS web console at [Basic] > [Administrative Access] > [System Account] > [Rebrand Clients], custom .ico icon file cannot be uploaded. The page refresh to the [Administrative Access] page, but the icon file was not uploaded.

Further checking on the CBS context log (e.g. cbs_context_YYYY-MM-SS.log) on the AhsayCBS server, the following error message is displayed:

Date h:mm:ss org.apache.catalina.core.ApplicationContext log
SEVERE:
java.lang.NullPointerException
at com.ahsay.cbs.system.config.accounts.EditSysUser$5.b(Unknown Source)
at com.ahsay.cbs.system.config.accounts.EditSysUser$5.a(Unknown Source)
at com.ahsay.cbs.system.config.accounts.EditSysUser$5.a(Unknown Source)
at com.ahsay.cbs.system.config.accounts.EditSysUser$5.a(Unknown Source)
at com.ahsay.cbs.system.config.accounts.EditSysUser$5.a(Unknown Source)
at com.ahsay.cbs.system.config.accounts.EditSysUser$5.execute(Unknown Source)