fix: Content banner

lib/widgets/content_banner.dart

@@ -42,11 +42,19 @@ class ContentBanner extends StatelessWidget {
             bottom: 0,
             child: Opacity(
               opacity: opacity,
-              child: LayoutBuilder(
-                  builder: (BuildContext context, BoxConstraints constraints) {
-                return Hero(
+              child: Hero(
                 tag: heroTag,
-                  child: MxcImage(
+                child: mxContent == null
+                    ? Center(
+                        child: Icon(
+                          defaultIcon,
+                          color: Theme.of(context)
+                              .colorScheme
+                              .onSecondaryContainer,
+                          size: 128,
+                        ),
+                      )
+                    : MxcImage(
                         key: Key(mxContent?.toString() ?? 'NoKey'),
                         uri: mxContent,
                         animated: true,
@@ -54,8 +62,7 @@ class ContentBanner extends StatelessWidget {
                         height: 400,
                         width: 800,
                       ),
-                );
-              }),
+              ),
             ),
           ),
           if (onEdit != null)