[CQ] fix nullability problems for flutter/android (#8295)

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

See #8291.

If we pursue #8292, we
could mark `src/io/flutter/android/` 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/FlutterMessages.java

@@ -23,7 +23,7 @@ public class FlutterMessages {
   private FlutterMessages() {
   }
 
-  public static void showError(String title, String message, @Nullable Project project) {
+  public static void showError(String title, @NotNull String message, @Nullable Project project) {
     Notifications.Bus.notify(
       new Notification(FLUTTER_NOTIFICATION_GROUP_ID,
                        title,

flutter-idea/src/io/flutter/android/AndroidEmulator.java

@@ -27,7 +27,7 @@
 import java.lang.reflect.Method;
 
 public class AndroidEmulator {
-  
+
   @NotNull final AndroidSdk androidSdk;
   @NotNull final String id;
   ProcessAdapter listener;
@@ -86,7 +86,9 @@ public void onTextAvailable(@NotNull ProcessEvent event, @NotNull Key outputType
         }
 
         public void processTerminated(@NotNull ProcessEvent event) {
-          process.removeProcessListener(listener);
+          if (listener != null) {
+            process.removeProcessListener(listener);
+          }
           final int exitCode = event.getExitCode();
           if (exitCode != 0) {
             final String message = stdout.isEmpty()
@@ -100,6 +102,7 @@ public void processTerminated(@NotNull ProcessEvent event) {
       process.startNotify();
     }
     catch (ExecutionException | RuntimeException e) {
+      //noinspection DataFlowIssue
       FlutterMessages.showError("Error Opening Emulator", e.toString(), androidSdk.project);
     }
   }

flutter-idea/src/io/flutter/android/AndroidSdk.java

@@ -110,7 +110,7 @@ public void onTextAvailable(@NotNull ProcessEvent event, @NotNull Key outputType
       }
 
       final Integer exitCode = process.getExitCode();
-      if (exitCode == null || process.getExitCode() != 0) {
+      if (exitCode == null || exitCode != 0) {
         return Collections.emptyList();
       }
 

flutter-idea/src/io/flutter/android/IntelliJAndroidSdk.java

@@ -49,18 +49,25 @@ public VirtualFile getHome() {
    * Changes the project's Java SDK to this one.
    */
   public void setCurrent(@NotNull Project project) {
-    assert ApplicationManager.getApplication().isWriteAccessAllowed();
+    var application = ApplicationManager.getApplication();
+    assert application != null;
+    assert application.isWriteAccessAllowed();
 
     final ProjectRootManager roots = ProjectRootManager.getInstance(project);
-    roots.setProjectSdk(sdk);
+    if (roots != null) {
+      roots.setProjectSdk(sdk);
+    }
   }
 
   /**
    * Returns the Java SDK in the project's configuration, or null if not an Android SDK.
    */
   @Nullable
   public static IntelliJAndroidSdk fromProject(@NotNull Project project) {
-    final Sdk candidate = ProjectRootManager.getInstance(project).getProjectSdk();
+    var manager = ProjectRootManager.getInstance(project);
+    if (manager == null) return null;
+
+    final Sdk candidate = manager.getProjectSdk();
     return fromSdk(candidate);
   }
 
@@ -89,7 +96,7 @@ public static IntelliJAndroidSdk fromEnvironment() {
   @Nullable
   public static IntelliJAndroidSdk fromHome(VirtualFile file) {
     for (IntelliJAndroidSdk candidate : findAll()) {
-      if (Objects.equals(file, candidate.getHome())) {
+      if (candidate != null && Objects.equals(file, candidate.getHome())) {
         return candidate;
       }
     }
@@ -100,7 +107,7 @@ public static IntelliJAndroidSdk fromHome(VirtualFile file) {
   /**
    * Returns the best value of the Android SDK location to use, including possibly querying flutter tools for it.
    */
-  public static String chooseAndroidHome(@Nullable Project project, boolean askFlutterTools) {
+  public static @Nullable String chooseAndroidHome(@Nullable Project project, boolean askFlutterTools) {
     if (project == null) {
       return EnvironmentUtil.getValue("ANDROID_HOME");
     }
@@ -130,10 +137,13 @@ public static String chooseAndroidHome(@Nullable Project project, boolean askFlu
   @NotNull
   private static List<IntelliJAndroidSdk> findAll() {
     final List<IntelliJAndroidSdk> result = new ArrayList<>();
-    for (Sdk sdk : ProjectJdkTable.getInstance().getAllJdks()) {
-      final IntelliJAndroidSdk candidate = IntelliJAndroidSdk.fromSdk(sdk);
-      if (candidate != null) {
-        result.add(candidate);
+    var jdkTable = ProjectJdkTable.getInstance();
+    if (jdkTable != null) {
+      for (Sdk sdk : jdkTable.getAllJdks()) {
+        final IntelliJAndroidSdk candidate = IntelliJAndroidSdk.fromSdk(sdk);
+        if (candidate != null) {
+          result.add(candidate);
+        }
       }
     }
     return result;