EDIT

This happens sometimes as is related to the mix of e3 and e4 views.
If this happens you can only force quit the taxeditor.

!SESSION 2017-10-03 13:39:04.510 -----------------------------------------------
eclipse.buildId=unknown
java.version=1.8.0_121
java.vendor=Oracle Corporation
BootLoader constants: OS=win32, ARCH=x86_64, WS=win32, NL=en
Framework arguments:  -Dserver_port=58080
Command-line arguments:  -os win32 -ws win32 -arch x86_64 -data @user.home/.cdmLibrary -Dserver_port=58080

This is a continuation of log file C:\Users\a.mueller\.cdmLibrary\.metadata\.bak_9.log
Created Time: 2017-10-03 13:53:32.090

!ENTRY eu.taxeditor 4 0 2017-10-03 13:53:32.090
!MESSAGE Could not get conversation for AbstractFormSection. There is an error in the implementation. There should have been an active editor but it wasn't
!STACK 0
java.lang.IllegalArgumentException
    at eu.etaxonomy.taxeditor.ui.element.AbstractFormSection.getConversationHolder(AbstractFormSection.java:590)
    at eu.etaxonomy.taxeditor.ui.section.AbstractCdmDetailElement.getConversationHolder(AbstractCdmDetailElement.java:277)
    at eu.etaxonomy.taxeditor.ui.section.occurrence.media.MediaSpecimenGeneralDetailElement.createControls(MediaSpecimenGeneralDetailElement.java:83)
    at eu.etaxonomy.taxeditor.ui.section.occurrence.media.MediaSpecimenGeneralDetailElement.createControls(MediaSpecimenGeneralDetailElement.java:1)
    at eu.etaxonomy.taxeditor.ui.section.AbstractCdmDetailElement.updateContent(AbstractCdmDetailElement.java:107)
    at eu.etaxonomy.taxeditor.ui.section.AbstractCdmDetailElement.setEntity(AbstractCdmDetailElement.java:94)
    at eu.etaxonomy.taxeditor.ui.section.AbstractCdmDetailSection.setEntity(AbstractCdmDetailSection.java:125)
    at eu.etaxonomy.taxeditor.view.detail.CdmSectionPart.setFormInput(CdmSectionPart.java:147)
    at org.eclipse.ui.forms.ManagedForm.setInput(ManagedForm.java:210)
    at eu.etaxonomy.taxeditor.view.e4.AbstractCdmDataViewerE4.refresh(AbstractCdmDataViewerE4.java:139)
    at eu.etaxonomy.taxeditor.view.e4.AbstractCdmDataViewerE4.setInput(AbstractCdmDataViewerE4.java:124)
    at eu.etaxonomy.taxeditor.view.e4.details.DetailsViewerE4.setInput(DetailsViewerE4.java:163)
    at eu.etaxonomy.taxeditor.view.e4.AbstractCdmEditorPartE4.showViewer(AbstractCdmEditorPartE4.java:181)
    at eu.etaxonomy.taxeditor.view.e4.details.DetailsPartE4.selectionChanged_internal(DetailsPartE4.java:78)