refactor: improve error message when sniper printer is set after building model

src/main/java/spoon/support/sniper/SniperJavaPrettyPrinter.java

@@ -90,7 +90,9 @@ public SniperJavaPrettyPrinter(Environment env) {
 	private ChangeCollector getChangeCollector() {
 		ChangeCollector changeCollector = ChangeCollector.getChangeCollector(env);
 		if (changeCollector == null) {
-			throw new SpoonException(ChangeCollector.class.getSimpleName() + " was not attached to the Environment");
+			throw new SpoonException(ChangeCollector.class.getSimpleName() + " was not attached to the Environment. "
+					+ "This typically means that the Sniper printer was set after building the model. "
+					+ "It must be set before building the model.");
 		}
 		return changeCollector;
 	}

src/test/java/spoon/test/prettyprinter/TestSniperPrinter.java

@@ -386,6 +386,34 @@ public void testPrintTypesThrowsWhenPassedTypesFromMultipleCompilationUnits() {
 		}
 	}
 
+	@Test
+	public void testCalculateCrashesWithInformativeMessageWhenSniperPrinterSetAfterModelBuild() {
+		// contract: The sniper printer must be set before building the model, and the error message
+		// one gets when this has not been done should say so.
+
+		Launcher launcher = new Launcher();
+		launcher.addInputResource(getResourcePath("visibility.YamlRepresenter"));
+
+		// build model, then set sniper
+		launcher.buildModel();
+		launcher.getEnvironment().setPrettyPrinterCreator(
+				() -> new SniperJavaPrettyPrinter(launcher.getEnvironment()));
+
+		CtCompilationUnit cu = launcher.getFactory().CompilationUnit().getMap().values().stream()
+				.findFirst().get();
+
+		// crash because sniper was set after model was built, and so the ChangeCollector was not
+		// attached to the environment
+		try {
+			launcher.createPrettyPrinter().calculate(cu, cu.getDeclaredTypes());
+		} catch (SpoonException e) {
+			assertThat(e.getMessage(), containsString(
+					"This typically means that the Sniper printer was set after building the model."));
+			assertThat(e.getMessage(), containsString(
+					"It must be set before building the model."));
+		}
+	}
+
 	@Test
 	public void testNewlineInsertedBetweenCommentAndTypeMemberWithAddedModifier() {
 		assumeNotWindows(); // FIXME Make test case pass on Windows