[CQ] fix nullability problems for flutter/propertyEditor (#8305)

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

See #8291.

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


---

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portions to this PR.

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Author: pq

flutter-idea/src/io/flutter/propertyeditor/PropertyEditorViewFactory.java

@@ -96,7 +96,7 @@ public void stateChanged(@NotNull ToolWindowManager toolWindowManager, @NotNull
     Disposer.register(toolWindow.getDisposable(), connection);
   }
 
-  private void checkDockedUnpinnedAndCreateContent(@NotNull Project project, ToolWindow toolWindow, boolean forceLoad) {
+  private void checkDockedUnpinnedAndCreateContent(@NotNull Project project, @NotNull ToolWindow toolWindow, boolean forceLoad) {
     final Boolean isDockedUnpinned = toolWindow.getType().equals(ToolWindowType.DOCKED) && toolWindow.isAutoHide();
 
     // If this is the first time we are loading the content, force a load even if docked unpinned state matches.