Werk #2774: Fix exception when deleting certain types of MIBs

Component	Setup
Title	Fix exception when deleting certain types of MIBs
Date	Nov 6, 2015
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	1.2.7i4 Checkmk Community, Checkmk Pro, Checkmk Ultimate MT

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