Don't try to remove elements that are not in the tree #3525

Moving an item from one project to another in the Project Explorer,
might break the tree structure, resulting in a phantom tree item in the
source project.

This is because the Project Explorer provides an extension point that
allows modifications by external projects (here JDT). When an element is
moved, its old position in the tree viewer is removed and its new
position added.

This removal is done both by the Platform and JDT. But when the element
is removed a second time, it no longer exists in the tree and instead a
dummy value is created.

To reproduce:

- Create a new project "Project A" and "Project B"
- Create a new text file "test" in "Project A"
- Move "test" to "Project B"

Closes #3525

Author: ptziegler

bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/AbstractTreeViewer.java

@@ -2062,6 +2062,10 @@ protected void internalRemove(Object[] elementsOrPaths) {
 				return;
 			}
 
+			if (findItem(element) == null) {
+				continue;
+			}
+
 			boolean continueOuter = false;
 			if (getItemsLimit() > 0) {
 				Widget[] itemsOfElement = internalFindItems(element);