Fix content type values being null after deserializing

When we restore the window state from a JSON file, the content type
field may be null if the GSON library was not able to map it to one
of the enum values.

Igalia#1644 changed the type of the
WindowState.panelType field from integer to enum. This meant that
the serialized values did not match the new type, causing the
restored field to unexpectedly become NULL. This caused a crash.

The solution is to change the name of the field to
WindowState.contentType so it will not cause a clash when serializing
and deserializing. Additionally, this PR adds checks to ensure that
the restored value is not NULL.

Fixes Igalia#1648

app/src/common/shared/com/igalia/wolvic/ui/widgets/Windows.java

@@ -98,7 +98,8 @@ class WindowState {
         int textureHeight;
         float worldWidth;
         int tabIndex = -1;
-        ContentType panelType = ContentType.WEB_CONTENT;
+        // NOTE: Enum values may be null when deserialized by GSON.
+        ContentType contentType = ContentType.WEB_CONTENT;
 
         public void load(@NonNull WindowWidget aWindow, WindowsState aState, int aTabIndex) {
             WidgetPlacement widgetPlacement;
@@ -118,10 +119,10 @@ public void load(@NonNull WindowWidget aWindow, WindowsState aState, int aTabInd
             worldWidth = widgetPlacement.worldWidth;
             tabIndex = aTabIndex;
             if (aWindow.isNativeContentVisible()) {
-                panelType = aWindow.getSelectedPanel();
+                contentType = aWindow.getSelectedPanel();
 
             } else {
-                panelType = ContentType.WEB_CONTENT;
+                contentType = ContentType.WEB_CONTENT;
             }
         }
     }
@@ -171,9 +172,10 @@ public enum ContentType {
         DOWNLOADS(UrlUtils.ABOUT_DOWNLOADS),
         ADDONS(UrlUtils.ABOUT_ADDONS),
         NOTIFICATIONS(UrlUtils.ABOUT_NOTIFICATIONS);
-        public final String URL;
 
-        ContentType(String url) {
+        @NonNull
+        public final String URL;
+        ContentType(@NonNull String url) {
             this.URL = url;
         }
     }
@@ -365,13 +367,16 @@ private WindowWidget addRestoredWindow(@NonNull WindowState aState, @Nullable Se
         if (aSession != null) {
             aSession.setActive(true);
         }
+        if (aState.contentType == null) {
+            aState.contentType = ContentType.WEB_CONTENT;
+        }
         WindowWidget newWindow = createWindow(aSession);
         newWindow.getPlacement().width = aState.textureWidth;
         newWindow.getPlacement().height = aState.textureHeight;
         newWindow.getPlacement().worldWidth = aState.worldWidth;
         placeWindow(newWindow, aState.placement);
         if (newWindow.getSession() != null) {
-            switch (aState.panelType) {
+            switch (aState.contentType) {
                 case BOOKMARKS:
                     newWindow.getSession().loadUri(UrlUtils.ABOUT_BOOKMARKS);
                     break;