Werk #8781: Fix traceback for builtin icon visibility

Component	Setup
Title	Fix traceback for builtin icon visibility
Date	Jul 16, 2019
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	2.0.0i1 Checkmk Community, Checkmk Pro, Checkmk Ultimate MT 1.6.0b4 Checkmk Community, Checkmk Pro, Checkmk Ultimate MT 1.5.0p20 Checkmk Community, Checkmk Pro, Checkmk Ultimate MT

If the "Builtin icon visibility" was configured for download_agent_output, but not for download_snmp_walk the action menu showed a traceback.

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