Werk #8844: Fix TypeError when sorting view by some columns

Component	User interface
Title	Fix TypeError when sorting view by some columns
Date	Aug 1, 2019
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	2.0.0i1 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME) 1.6.0b5 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

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