refactor(core/rust): don't panic if Rust layout is unavailable

diff --git a/core/embed/rust/src/ui/layout/obj.rs b/core/embed/rust/src/ui/layout/obj.rs
index 248348fb..a1ff12c2 100644
--- a/core/embed/rust/src/ui/layout/obj.rs
+++ b/core/embed/rust/src/ui/layout/obj.rs
@@ -212,6 +212,8 @@ struct LayoutObjInner {
     button_request: Option<ButtonRequest>,
 }
 
+const NO_LAYOUT: Error = Error::RuntimeError(c"No layout");
+
 impl LayoutObjInner {
     /// Create a new `LayoutObj`, wrapping a root component.
     #[inline(never)]
@@ -229,7 +231,7 @@ impl LayoutObjInner {
         };
 
         // invoke the initial placement
-        new.root_mut().place();
+        new.root_mut()?.place();
         // cause a repaint pass to update the number of pages
         let msg = new.obj_event(Event::RequestPaint);
         assert!(matches!(msg, Ok(s) if s == Obj::const_none()));
@@ -247,22 +249,23 @@ impl LayoutObjInner {
         self.timer_fn = timer_fn;
     }
 
-    fn root(&self) -> &impl Deref<Target = dyn LayoutMaybeTrace> {
-        unwrap!(self.root.as_ref())
+    fn root(&self) -> Result<&impl Deref<Target = dyn LayoutMaybeTrace>, Error> {
+        self.root.as_ref().ok_or(NO_LAYOUT)
     }
 
-    fn root_mut(&mut self) -> &mut impl DerefMut<Target = dyn LayoutMaybeTrace> {
-        unwrap!(self.root.as_mut())
+    fn root_mut(&mut self) -> Result<&mut impl DerefMut<Target = dyn LayoutMaybeTrace>, Error> {
+        self.root.as_mut().ok_or(NO_LAYOUT)
     }
 
-    fn obj_request_repaint(&mut self) {
+    fn obj_request_repaint(&mut self) -> Result<(), Error> {
         self.repaint = Repaint::Full;
         let mut event_ctx = EventCtx::new();
-        let paint_msg = self.root_mut().event(&mut event_ctx, Event::RequestPaint);
+        let paint_msg = self.root_mut()?.event(&mut event_ctx, Event::RequestPaint);
         // paint_msg must not change the state
         assert!(paint_msg.is_none());
         // there must be no timers set
         assert!(event_ctx.pop_timer().is_none());
+        Ok(())
     }
 
     /// Run an event pass over the component tree. After the traversal, any
@@ -270,7 +273,7 @@ impl LayoutObjInner {
     /// in case the timer callback raises or one of the components returns
     /// an error, `Ok` with the message otherwise.
     fn obj_event(&mut self, event: Event) -> Result<Obj, Error> {
-        let root = unwrap!(self.root.as_mut());
+        let root = self.root.as_mut().ok_or(NO_LAYOUT)?;
 
         // Get the event context ready for a new event
         self.event_ctx.clear();
@@ -295,7 +298,7 @@ impl LayoutObjInner {
 
         // Check if we should repaint next time
         if self.event_ctx.needs_repaint_root() {
-            self.obj_request_repaint();
+            self.obj_request_repaint()?;
         } else if self.event_ctx.needs_repaint() && self.repaint == Repaint::None {
             self.repaint = Repaint::Partial;
         }
@@ -329,7 +332,7 @@ impl LayoutObjInner {
 
         if self.repaint != Repaint::None {
             self.repaint = Repaint::None;
-            self.root_mut().paint().map(|_| true)
+            self.root_mut()?.paint().map(|_| true)
         } else {
             Ok(false)
         }
@@ -339,7 +342,7 @@ impl LayoutObjInner {
     /// with each piece of tracing information. Panics in case the callback
     /// raises an exception.
     #[cfg(feature = "ui_debug")]
-    fn obj_trace(&self, callback: Obj) {
+    fn obj_trace(&self, callback: Obj) -> Result<(), Error> {
         use crate::trace::JsonTracer;
 
         let mut tracer = JsonTracer::new(|text: &str| {
@@ -354,9 +357,9 @@ impl LayoutObjInner {
         // to claim that it implements `Trace`, and we also can't upcast it to
         // `&dyn Trace` because trait upcasting is unstable.
         // Luckily, calling `root.trace()` works perfectly fine in spite of the above.)
-        tracer.root(&|t| {
-            self.root().trace(t);
-        });
+        let root = self.root()?;
+        tracer.root(&|t| root.trace(t));
+        Ok(())
     }
 
     fn obj_page_count(&self) -> Obj {
@@ -375,7 +378,7 @@ impl LayoutObjInner {
     }
 
     fn obj_return_value(&self) -> Result<Obj, Error> {
-        self.root()
+        self.root()?
             .value()
             .cloned()
             .unwrap_or(Ok(Obj::const_none()))
@@ -651,7 +654,7 @@ extern "C" fn ui_layout_paint(this: Obj) -> Obj {
 extern "C" fn ui_layout_request_complete_repaint(this: Obj) -> Obj {
     let block = || {
         let this: Gc<LayoutObj> = this.try_into()?;
-        this.inner_mut().obj_request_repaint();
+        this.inner_mut().obj_request_repaint()?;
         Ok(Obj::const_none())
     };
     unsafe { util::try_or_raise(block) }
@@ -703,7 +706,7 @@ pub extern "C" fn ui_debug_layout_type() -> &'static Type {
 extern "C" fn ui_layout_trace(this: Obj, callback: Obj) -> Obj {
     let block = || {
         let this: Gc<LayoutObj> = this.try_into()?;
-        this.inner_mut().obj_trace(callback);
+        this.inner_mut().obj_trace(callback)?;
         Ok(Obj::const_none())
     };
     unsafe { util::try_or_raise(block) }