Test test case testGetImageRegistryFromAdditionalDisplay() in
org.eclipse.ui.tests.api.WorkbenchPluginTest randomly fails because of a
race condition. A display is initialized in another thread, but the
initialization logic may access the value before it has actually been
set by the other thread. This is fixed by reordering the instructions.

Fixes https://github.com/eclipse-platform/eclipse.platform.ui/issues/1891