Backport to 7.9 - Cancel on close all tabs dialog doesn't work

When a document cannot be saved due to validation errors, the close all tabs dialog stops working. Here is the reproduction path, from the forum.

I could reproduce it on CMS 2.26.19

We could reproduce this behaviour:

in cms:
1.) create a document that contains a required field but leave the required field empty (= invalid)

2.) right click on the document's tab and choose "Close All"
-> the "Close all tabs" window opens

3.) click "Save All" (which of course leads to an error, since the document is not valid)

4.) now click "Cancel" (to get back to your document) /"Discard All"
-> nothing happens.
-> only way to close the window is by click on "X"

5.) error log:
"Cancel button:"
[INFO] [talledLocalContainer] 27.04.2015 10:01:11 ERROR http-bio-8080-exec-5 [DefaultExceptionMapper.internalMap:129] Unexpected error occurred
[INFO] [talledLocalContainer] org.apache.wicket.WicketRuntimeException: Component 'dialog:content:form:buttons:3:button' has been removed from page.
"Discard All" button:
[INFO] [talledLocalContainer] 27.04.2015 10:02:05 ERROR http-bio-8080-exec-8 [DefaultExceptionMapper.internalMap:129] Unexpected error occurred
[INFO] [talledLocalContainer] org.apache.wicket.WicketRuntimeException: Component 'dialog:content:form:buttons:1:button' has been removed from page.