[CQ] API: migrate off unstable getInstanceOrNull (#8121)

This method is marked as unstable and slated for possible removal.


![image](https://github.com/user-attachments/assets/feb9c87a-0cc4-44b1-8b40-509b21c48143)

That said, this replacement doesn't totally preserve the current
semantics.

@helin24: is there any reason we don't want to prompt a call to the
application manager here?

Specifically, we'll now cause this to happen (if `ourInstance` is
`null`):

```java
(ProgressManager)ApplicationManager.getApplication().getService(ProgressManager.class);
```

See: ##7718


---

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portions to this PR.

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Author: pq

flutter-idea/src/io/flutter/view/FlutterView.java

@@ -198,10 +198,8 @@ private void addBrowserInspectorViewContent(FlutterApp app,
             });
           }));
       };
-      final ProgressManager progressManager = ProgressManager.getInstanceOrNull();
-      if (progressManager != null) {
-        progressManager.runProcess(task, new EmptyProgressIndicator());
-      }
+      final ProgressManager progressManager = ProgressManager.getInstance();
+      progressManager.runProcess(task, new EmptyProgressIndicator());
 
       toolWindow.setTitleActions(List.of(new RefreshToolWindowAction(TOOL_WINDOW_ID)));
     }