[CQ] fix nullability problems for flutter/samples (#8307)

Fix nullability problems in `src/io/flutter/samples/`.

See #8291.

If we pursue #8292, we
could mark `src/io/flutter/samples/` as null-"clean".


---

- [x] I’ve reviewed the contributor guide and applied the relevant
portions to this PR.

<details>
  <summary>Contribution guidelines:</summary><br>

- See our [contributor
guide]([https://github.com/dart-lang/sdk/blob/main/CONTRIBUTING.md](https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview)
for general expectations for PRs.
- Larger or significant changes should be discussed in an issue before
creating a PR.
- Dart contributions to our repos should follow the [Dart style
guide](https://dart.dev/guides/language/effective-dart) and use `dart
format`.
- Java and Kotlin contributions should strive to follow Java and Kotlin
best practices
([discussion](#8098)).
</details>

Author: pq

flutter-idea/src/io/flutter/samples/DartDocumentUtils.java

@@ -72,7 +72,7 @@ public static List<String> getDartdocFor(@NotNull Document document, @NotNull Da
     return lines;
   }
 
-  private static String getLine(Document document, int line) {
+  private static String getLine(@NotNull Document document, int line) {
     return document.getText(new TextRange(document.getLineStartOffset(line - 1), document.getLineEndOffset(line - 1)));
   }
 }

flutter-idea/src/io/flutter/samples/FlutterSampleNotificationProvider.java

@@ -7,14 +7,12 @@
 
 import com.google.common.annotations.VisibleForTesting;
 import com.intellij.ide.BrowserUtil;
-import com.intellij.openapi.application.ApplicationManager;
 import com.intellij.openapi.editor.Document;
 import com.intellij.openapi.editor.Editor;
 import com.intellij.openapi.editor.colors.EditorColors;
 import com.intellij.openapi.fileEditor.FileEditor;
 import com.intellij.openapi.fileEditor.TextEditor;
 import com.intellij.openapi.project.Project;
-import com.intellij.openapi.util.Computable;
 import com.intellij.openapi.vfs.VirtualFile;
 import com.intellij.psi.PsiDocumentManager;
 import com.intellij.psi.PsiFile;
@@ -25,6 +23,7 @@
 import com.jetbrains.lang.dart.psi.DartClass;
 import icons.FlutterIcons;
 import io.flutter.sdk.FlutterSdk;
+import io.flutter.utils.OpenApiUtils;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
@@ -86,10 +85,9 @@ private EditorNotificationPanel createPanelForSamples(@NotNull FileEditor fileEd
     }
 
     // Run the code to query the document in a read action.
-    final List<FlutterSample> samples = ApplicationManager.getApplication().
-      runReadAction((Computable<List<FlutterSample>>)() -> {
-        return getSamplesFromDoc(flutterPackagePath, document, filePath);
-      });
+    final List<FlutterSample> samples = OpenApiUtils.safeRunReadAction(() -> {
+      return getSamplesFromDoc(flutterPackagePath, document, filePath);
+    });
 
     if (samples != null && !samples.isEmpty()) {
       return new FlutterSampleActionsPanel(samples);
@@ -101,8 +99,11 @@ private EditorNotificationPanel createPanelForSamples(@NotNull FileEditor fileEd
   private List<FlutterSample> getSamplesFromDoc(@NotNull String flutterPackagePath, @NotNull Document document, @NotNull String filePath) {
     final List<FlutterSample> samples = new ArrayList<>();
 
+    PsiDocumentManager documentManager = PsiDocumentManager.getInstance(project);
+    if (documentManager == null) return samples;
+
     // Find all candidate class definitions.
-    final PsiFile psiFile = PsiDocumentManager.getInstance(project).getPsiFile(document);
+    final PsiFile psiFile = documentManager.getPsiFile(document);
     assert (psiFile != null);
 
     final DartClass[] classes = PsiTreeUtil.getChildrenOfType(psiFile, DartClass.class);
@@ -114,6 +115,7 @@ private List<FlutterSample> getSamplesFromDoc(@NotNull String flutterPackagePath
     // "/// {@tool dartpad ...}".
 
     for (DartClass declaration : classes) {
+      if (declaration == null) continue;
       final String name = declaration.getName();
       if (name == null || name.startsWith("_")) {
         continue;
@@ -157,7 +159,7 @@ public static boolean containsDartdocFlutterSample(@NotNull List<String> lines)
     }
 
     for (String line : lines) {
-      if (DARTPAD_TOOL_PATTERN.matcher(line).find()) {
+      if (line != null && DARTPAD_TOOL_PATTERN.matcher(line).find()) {
         return true;
       }
     }
@@ -168,24 +170,27 @@ public static boolean containsDartdocFlutterSample(@NotNull List<String> lines)
 
 class FlutterSampleActionsPanel extends EditorNotificationPanel {
   FlutterSampleActionsPanel(@NotNull List<FlutterSample> samples) {
+    //noinspection DataFlowIssue
     super(EditorColors.GUTTER_BACKGROUND);
 
     icon(FlutterIcons.Flutter);
     text("View example on flutter.dev");
 
     for (int i = 0; i < samples.size(); i++) {
       if (i != 0) {
+        //noinspection DataFlowIssue
         myLinksPanel.add(new JSeparator(SwingConstants.VERTICAL));
       }
 
       final FlutterSample sample = samples.get(i);
+      assert sample != null;
 
       final HyperlinkLabel label = createActionLabel(sample.getClassName(), () -> browseTo(sample));
       label.setToolTipText(sample.getHostedDocsUrl());
     }
   }
 
-  private void browseTo(FlutterSample sample) {
+  private void browseTo(@NotNull FlutterSample sample) {
     BrowserUtil.browse(sample.getHostedDocsUrl());
   }
 }