Fix NPE when Find action is unavailable in Expressions view

The Find action may be unavailable when the focus is outside the
Expressions view, which leads to an NPE in fillContextMenu.

This adds a null and enablement check for the Find action to resolve
the problem.

Author: mx990

debug/org.eclipse.debug.ui/ui/org/eclipse/debug/internal/ui/views/expression/ExpressionView.java

@@ -84,8 +84,10 @@ protected void fillContextMenu(IMenuManager menu) {
 		IAction action;
 		if (DebugPlugin.getDefault().getExpressionManager().getExpressions().length > 0) {
 			action = getAction(FIND_ACTION);
-			action.setImageDescriptor(DebugPluginImages.getImageDescriptor(IDebugUIConstants.IMG_FIND_ACTION));
-			menu.add(action);
+			if (action != null && action.isEnabled()) {
+				action.setImageDescriptor(DebugPluginImages.getImageDescriptor(IDebugUIConstants.IMG_FIND_ACTION));
+				menu.add(action);
+			}
 		}
 		ChangeVariableValueAction changeValueAction = (ChangeVariableValueAction)getAction("ChangeVariableValue"); //$NON-NLS-1$
 		if (changeValueAction.isApplicable()) {