Fix Widget is disposed exception in VisibleLinesTracker

Author: wahlbrink

bundles/org.eclipse.jface.text/src/org/eclipse/jface/text/source/VisibleLinesTracker.java

@@ -91,7 +91,7 @@ private Map<Integer, Rectangle> getVisibleLineBounds() {
 	 */
 	static void track(ITextViewer viewer, Consumer<StyledText> handler) {
 		StyledText textWidget= viewer != null ? viewer.getTextWidget() : null;
-		if (textWidget == null) {
+		if (textWidget == null || textWidget.isDisposed()) {
 			return;
 		}
 		VisibleLinesTracker tracker= (VisibleLinesTracker) textWidget.getData(DATA_KEY);
@@ -111,7 +111,7 @@ static void track(ITextViewer viewer, Consumer<StyledText> handler) {
 	 */
 	static void untrack(ITextViewer viewer, Consumer<StyledText> handler) {
 		StyledText textWidget= viewer != null ? viewer.getTextWidget() : null;
-		if (textWidget == null) {
+		if (textWidget == null || textWidget.isDisposed()) {
 			return;
 		}
 		VisibleLinesTracker tracker= (VisibleLinesTracker) textWidget.getData(DATA_KEY);