[Backport 12.6] Can't create content from channel manager with channel in project

Projects does seem to work well with create content:

1. Create a new project and add a channel
2. Navigate to channel and change “For Project” drop down to new project
3. Find a component with a create content button (I used news component in HAP)
4. Click create content button. Fill some fields
5. Click Save. First click gives message “An unexpected error occurred, please try again “. Second and further clicks give message “ERROR_NOT_EDITABLE”.

26.11.2018 14:58:41 +0100 [catalina-exec-13] WARN [org.onehippo.cms.channelmanager.content.document.util.EditingUtils.getEditableDocumentNode():222] Failed to obtain draft for user 'admin'.
org.hippoecm.repository.api.WorkflowException: Action 'obtainEditableInstance' is not allowed for project with id 'vb0hj'.
at com.onehippo.repository.wpm.project.documentworkflow.WpmDocumentWorkflowImpl.triggerAction(WpmDocumentWorkflowImpl.java:100) ~[hippo-addon-wpm-repository-1.6.0.jar:1.6.0]
at org.onehippo.repository.documentworkflow.DocumentWorkflowImpl.obtainEditableInstance(DocumentWorkflowImpl.java:224) ~[hippo-repository-workflow-5.6.0.jar:5.6.0]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_181]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_181]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_181]
at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_181]
at org.hippoecm.repository.impl.WorkflowManagerImpl$WorkflowInvocationHandler.invoke(WorkflowManagerImpl.java:373) ~[hippo-repository-engine-5.6.0.jar:5.6.0]
at com.sun.proxy.$Proxy211.obtainEditableInstance(Unknown Source) ~[?:?]
at org.onehippo.cms.channelmanager.content.document.util.EditingUtils.getEditableDocumentNode(EditingUtils.java:219) ~[hippo-addon-channel-manager-content-service-5.6.0.jar:5.6.0]
at org.onehippo.cms.channelmanager.content.document.DocumentsServiceImpl.updateEditableDocument(DocumentsServiceImpl.java:280) ~[hippo-addon-channel-manager-content-service-5.6.0.jar:5.6.0]
at org.onehippo.cms.channelmanager.content.ContentResource.lambda$updateEditableDocument$2(ContentResource.java:103) ~[hippo-addon-channel-manager-content-service-5.6.0