Fix SDK browsing NPE in FlutterSettingsConfigurable (#8102)

If no file is selected, the file chooser will return `null`. This guards
against that.

Fixes: #8069

Author: pq

flutter-idea/src/io/flutter/sdk/FlutterSettingsConfigurable.java

@@ -141,6 +141,7 @@ protected void textChanged(@NotNull final DocumentEvent e) {
   }
 
   private void createUIComponents() {
+    //noinspection DialogTitleCapitalization
     ExtendableTextComponent.Extension browseExtension =
       ExtendableTextComponent.Extension.create(
         AllIcons.General.OpenDisk,
@@ -149,7 +150,10 @@ private void createUIComponents() {
         () -> {
           FileChooserDescriptor descriptor = FileChooserDescriptorFactory.createSingleFolderDescriptor();
           VirtualFile file = FileChooser.chooseFile(descriptor, mySdkCombo, null, null);
-          mySdkCombo.setItem(file.getPath());
+          if (file != null) {
+            //noinspection DataFlowIssue (sure to be set before the extension is invoked)
+            mySdkCombo.setItem(file.getPath());
+          }
         });
     mySdkCombo = new ComboBox<>();
     mySdkCombo.setEditor(new BasicComboBoxEditor() {