[modern mode] fail open an image after update

Nobody has claimed this yet.

Hello,
I just made an update with the internal update manager and since then time I cannot open an image and receive following error message:

```
[ERROR] Exception during event handling:
[Event] org.scijava.ui.dnd.event.DropEvent
context = org.scijava.Context@3d43fe
consumed = false
display = null
modifiers = null
x = 187
y = 40
data = org.scijava.ui.awt.AWTDragAndDropData@68c9eeb9
successful = false
[Subscriber] org.scijava.ui.dnd.DefaultDragAndDropService [priority = 0.0]
[Method] protected void org.scijava.ui.dnd.DefaultDragAndDropService.onEvent(org.scijava.ui.dnd.event.DropEvent)
java.lang.NullPointerException
at sun.awt.dnd.SunDropTargetContextPeer.getTransferDataFlavors(SunDropTargetContextPeer.java:196)
at sun.awt.datatransfer.TransferableProxy.getTransferDataFlavors(TransferableProxy.java:65)
at java.awt.dnd.DropTargetContext$TransferableProxy.getTransferDataFlavors(DropTargetContext.java:344)
at org.scijava.ui.awt.AWTDragAndDropData.getMIMETypes(AWTDragAndDropData.java:109)
at org.scijava.ui.awt.AWTDragAndDropData.getMIMETypes(AWTDragAndDropData.java:54)
at org.scijava.ui.dnd.DragAndDropData.getMIMEType(DragAndDropData.java:81)
at org.scijava.ui.dnd.DragAndDropData.isSupported(DragAndDropData.java:57)
at org.scijava.ui.dnd.AbstractDragAndDropHandler.supportsData(AbstractDragAndDropHandler.java:56)
at org.scijava.ui.dnd.AbstractDragAndDropHandler.supportsData(AbstractDragAndDropHandler.java:63)
at org.scijava.ui.dnd.DragAndDropService.getHandler(DragAndDropService.java:115)
at org.scijava.ui.dnd.DragAndDropService.supports(DragAndDropService.java:60)
at org.scijava.ui.dnd.DefaultDragAndDropService.onEvent(DefaultDragAndDropService.java:90)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at org.scijava.event.DefaultEventService$ProxySubscriber.onEvent(DefaultEventService.java:301)
at org.scijava.event.DefaultEventService$ProxySubscriber.onEvent(DefaultEventService.java:275)
at org.bushe.swing.event.ThreadSafeEventService.publish(ThreadSafeEventService.java:971)
at org.scijava.event.DefaultEventBus.access$101(DefaultEventBus.java:57)
at org.scijava.event.DefaultEventBus$1.run(DefaultEventBus.java:191)
at org.scijava.thread.DefaultThreadService.invoke(DefaultThreadService.java:114)
at org.scijava.event.DefaultEventBus.publishNow(DefaultEventBus.java:185)
at org.scijava.event.DefaultEventBus.publishNow(DefaultEventBus.java:76)
at org.scijava.event.DefaultEventService.publish(DefaultEventService.java:105)
at org.scijava.ui.awt.AWTDropTargetEventDispatcher.drop(AWTDropTargetEventDispatcher.java:133)
at java.awt.dnd.DropTarget.drop(DropTarget.java:455)
at sun.awt.dnd.SunDropTargetContextPeer.processDropMessage(SunDropTargetContextPeer.java:538)
at sun.awt.X11.XDropTargetContextPeer.processDropMessage(XDropTargetContextPeer.java:184)
at sun.awt.dnd.SunDropTargetContextPeer$EventDispatcher.dispatchDropEvent(SunDropTargetContextPeer.java:852)
at sun.awt.dnd.SunDropTargetContextPeer$EventDispatcher.dispatchEvent(SunDropTargetContextPeer.java:776)
at sun.awt.dnd.SunDropTargetEvent.dispatch(SunDropTargetEvent.java:48)
at java.awt.Component.dispatchEventImpl(Component.java:4746)
at java.awt.Container.dispatchEventImpl(Container.java:2294)
at java.awt.Window.dispatchEventImpl(Window.java:2750)
at java.awt.Component.dispatchEvent(Component.java:4713)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.awt.EventQueue$3.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86)
at java.awt.EventQueue$4.run(EventQueue.java:731)
at java.awt.EventQueue$4.run(EventQueue.java:729)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)
```
ImageJ 2.0.0-rc-69/.52i
Java 1.8.0_172 [amb64]

Start at org.scijava.ui.awt.AWTDragAndDropData.getMIMETypes(), following the drop path through DragAndDropData.isSupported() and DefaultDragAndDropService.onEvent(). Reproduce the failure by dropping an image after the reported update; done means the drop no longer raises the NullPointerException and the image opens.

Written by the indexing model from the issue text.