[bugfix] Report null results instead of throwing NPE

Author: adamretter

exist-core/src/main/java/org/exist/client/QueryDialog.java

@@ -76,6 +76,7 @@
 import org.fife.ui.rtextarea.RTextScrollPane;
 import org.xmldb.api.base.Collection;
 import org.xmldb.api.base.CompiledExpression;
+import org.xmldb.api.base.ErrorCodes;
 import org.xmldb.api.base.Resource;
 import org.xmldb.api.base.ResourceIterator;
 import org.xmldb.api.base.ResourceSet;
@@ -591,6 +592,10 @@ public void run() {
                 tResult = System.currentTimeMillis() - t1;
                 runningContext.set(null);
 
+                if (result == null) {
+                    throw new XMLDBException(ErrorCodes.UNKNOWN_ERROR, "Query returned 'null' which it should never do, this is likely a bug that should be reported");
+                }
+
                 // jmfg: Is this still needed? I don't think so
                 writer = new StringWriter();
                 service.dump(compiled, writer);