EuiSearchBar throws error stemming from EuiMutationObserver

[cjcenizal]

CC @chandlerprall

You can repro this by opening the filters drodown in the docs.

Warning: Failed prop type: The prop `children` is marked as required in `EuiMutationObserver`, but its value is `null`.
    in EuiMutationObserver (created by EuiMutationObserver)
    in EuiMutationObserver (created by EuiPopover)
    in div (created by EuiPanel)
    in EuiPanel (created by EuiPopover)
    in div (created by FocusTrap)
    in FocusTrap (created by EuiPopover)
    in EuiPortal (created by EuiPopover)
    in div (created by EuiPopover)
    in EuiOutsideClickDetector (created by EuiPopover)
    in EuiPopover (created by FieldValueSelectionFilter)
    in FieldValueSelectionFilter (created by EuiSearchFilters)
    in div (created by EuiFilterGroup)
    in EuiFilterGroup (created by EuiSearchFilters)
    in EuiSearchFilters (created by EuiSearchBar)
    in div (created by EuiFlexItem)
    in EuiFlexItem (created by EuiSearchBar)
    in div (created by EuiFlexGroup)
    in EuiFlexGroup (created by EuiSearchBar)
    in EuiSearchBar (created by SearchBar)
    in div (created by EuiFlexItem)
    in EuiFlexItem (created by SearchBar)
    in div (created by EuiFlexGroup)
    in EuiFlexGroup (created by SearchBar)
    in SearchBar
    in div (created by GuideSection)
    in EuiErrorBoundary (created by GuideSection)
    in div (created by GuideSection)
    in GuideSection (created by Connect(GuideSection))
    in Connect(GuideSection)
    in GuidePage (created by component)
    in EuiErrorBoundary (created by component)
    in component (created by RouterContext)
    in div (created by AppView)
    in div (created by EuiPageBody)
    in EuiPageBody (created by AppView)
    in div (created by EuiPage)
    in EuiPage (created by AppView)
    in div (created by AppView)
    in AppView (created by Connect(AppView))
    in Connect(AppView) (created by RouterContext)
    in RouterContext (created by Router)
    in Router
    in Provider

[snide]

FWIW that dropdown is not respecting size as well. Might be related.