Polish XmlReportsWritingListener

- Set reportData to null when the test plan is finished to allow for
  garbage collection
- Rename method handleException to printException to better state what
  it's doing
- Inline message variables to printException for brevity

Issue: #86

junit-console/src/main/java/org/junit/gen5/console/tasks/XmlReportsWritingListener.java

@@ -49,11 +49,15 @@ public void testPlanExecutionStarted(TestPlan testPlan) {
 			Files.createDirectories(reportsDir.toPath());
 		}
 		catch (IOException e) {
-			String message = "Could not create report directory: " + reportsDir;
-			handleException(e, message);
+			printException("Could not create report directory: " + reportsDir, e);
 		}
 	}
 
+	@Override
+	public void testPlanExecutionFinished(TestPlan testPlan) {
+		this.reportData = null;
+	}
+
 	@Override
 	public void executionSkipped(TestIdentifier testIdentifier, String reason) {
 		reportData.markSkipped(testIdentifier, reason);
@@ -79,16 +83,15 @@ private void writeXmlReportSafely(TestIdentifier testIdentifier) {
 			new XmlReportWriter(reportData).writeXmlReport(testIdentifier, xmlFile);
 		}
 		catch (XMLStreamException | IOException e) {
-			String message = "Could not write file: " + xmlFile;
-			handleException(e, message);
+			printException("Could not write file: " + xmlFile, e);
 		}
 	}
 
 	private boolean isARoot(TestIdentifier testIdentifier) {
 		return !testIdentifier.getParentId().isPresent();
 	}
 
-	private void handleException(Exception exception, String message) {
+	private void printException(String message, Exception exception) {
 		out.println(message);
 		exception.printStackTrace(out);
 	}