Fix background color update for TemplatePreviewerFragment in CardTemplateEditor

The fragment's view was accessed to set the background on it but it
wasn't available at that moment resulting in an IllegalStateException.

The fragment's view wasn't available because although the transaction
that adds the fragment is done synchronously it only takes the fragment
to the parent's CURRENT lifecycle state(still too early in the lifecycle).

Author: lukstbit

AnkiDroid/src/main/java/com/ichi2/anki/CardTemplateEditor.kt

@@ -203,12 +203,11 @@ open class CardTemplateEditor : AnkiActivity(), DeckSelectionListener {
                 tags = note.tags,
                 fillEmpty = true
             )
-            val fragment = TemplatePreviewerFragment.newInstance(args)
+            val backgroundColor = ThemeUtils.getThemeAttrColor(this@CardTemplateEditor, R.attr.alternativeBackgroundColor)
+            val fragment = TemplatePreviewerFragment.newInstance(args, backgroundColor)
             supportFragmentManager.commitNow {
                 replace(R.id.fragment_container, fragment)
             }
-            val backgroundColor = ThemeUtils.getThemeAttrColor(this@CardTemplateEditor, R.attr.alternativeBackgroundColor)
-            fragment.requireView().setBackgroundColor(backgroundColor)
         }
     }
 

AnkiDroid/src/main/java/com/ichi2/anki/previewer/TemplatePreviewerFragment.kt

@@ -29,6 +29,7 @@ import com.ichi2.anki.cardviewer.CardMediaPlayer
 import com.ichi2.anki.snackbar.BaseSnackbarBuilderProvider
 import com.ichi2.anki.snackbar.SnackbarBuilder
 import com.ichi2.anki.utils.ext.sharedPrefs
+import com.ichi2.utils.BundleUtils.getNullableInt
 import kotlinx.coroutines.flow.launchIn
 import kotlinx.coroutines.flow.onEach
 
@@ -65,14 +66,28 @@ class TemplatePreviewerFragment :
         if (sharedPrefs().getBoolean("safeDisplay", false)) {
             view.findViewById<MaterialCardView>(R.id.webview_container).elevation = 0F
         }
+
+        arguments?.getNullableInt(ARG_BACKGROUND_OVERRIDE_COLOR)?.let { color ->
+            view.setBackgroundColor(color)
+        }
     }
 
     companion object {
         const val ARGS_KEY = "templatePreviewerArgs"
+        private const val ARG_BACKGROUND_OVERRIDE_COLOR = "arg_background_override_color"
 
-        fun newInstance(arguments: TemplatePreviewerArguments): TemplatePreviewerFragment {
+        /**
+         * @param backgroundOverrideColor optional color to be used as background on the root view
+         * of this fragment
+         */
+        fun newInstance(
+            arguments: TemplatePreviewerArguments,
+            backgroundOverrideColor: Int? = null
+        ): TemplatePreviewerFragment {
             return TemplatePreviewerFragment().apply {
-                this.arguments = bundleOf(ARGS_KEY to arguments)
+                val args = bundleOf(ARGS_KEY to arguments)
+                backgroundOverrideColor?.let { args.putInt(ARG_BACKGROUND_OVERRIDE_COLOR, backgroundOverrideColor) }
+                this.arguments = args
             }
         }
     }