xdg-toplevel: remove listeners before view destroy

Currently in handleUnmap() we call View.unmap() before removing
listeners. However View.unmap() may destroy the view before returning
if the transaction started doesn't have to wait on any configures.

To ensure that we don't try to remove listeners which have already been
free'd, do this before calling View.unmap().

 river/Root.zig        | 3 +--
 river/View.zig        | 3 +++
 river/XdgToplevel.zig | 4 ++--
 3 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/river/Root.zig b/river/Root.zig
index a74d0ee..3ea79cf 100644
--- a/river/Root.zig
+++ b/river/Root.zig
@@ -384,13 +384,12 @@ fn commitTransaction(self: *Self) void {
         var view_it = output.views.first;
         while (view_it) |view_node| {
             const view = &view_node.view;
+            view_it = view_node.next;
 
             if (view.destroying) {
-                view_it = view_node.next;
                 view.destroy();
                 continue;
             }
-            defer view_it = view_node.next;
 
             if (view.pending_serial != null and !view.shouldTrackConfigure()) continue;
 
diff --git a/river/View.zig b/river/View.zig
index 816c72e..5b57774 100644
--- a/river/View.zig
+++ b/river/View.zig
@@ -19,6 +19,7 @@ const Self = @This();
 
 const build_options = @import("build_options");
 const std = @import("std");
+const assert = std.debug.assert;
 const math = std.math;
 const os = std.os;
 const wlr = @import("wlroots");
@@ -486,7 +487,9 @@ pub fn map(self: *Self) void {
 pub fn unmap(self: *Self) void {
     log.debug("view '{s}' unmapped", .{self.getTitle()});
 
+    assert(!self.destroying);
     self.destroying = true;
+
     if (self.saved_buffers.items.len == 0) self.saveBuffers();
 
     if (self.opacity_timer != null) {
diff --git a/river/XdgToplevel.zig b/river/XdgToplevel.zig
index 96b618b..ca360c3 100644
--- a/river/XdgToplevel.zig
+++ b/river/XdgToplevel.zig
@@ -248,8 +248,6 @@ fn handleMap(listener: *wl.Listener(*wlr.XdgSurface), xdg_surface: *wlr.XdgSurfa
 fn handleUnmap(listener: *wl.Listener(*wlr.XdgSurface), xdg_surface: *wlr.XdgSurface) void {
     const self = @fieldParentPtr(Self, "unmap", listener);
 
-    self.view.unmap();
-
     // Remove listeners that are only active while mapped
     self.commit.link.remove();
     self.request_fullscreen.link.remove();
@@ -257,6 +255,8 @@ fn handleUnmap(listener: *wl.Listener(*wlr.XdgSurface), xdg_surface: *wlr.XdgSur
     self.request_resize.link.remove();
     self.set_title.link.remove();
     self.set_app_id.link.remove();
+
+    self.view.unmap();
 }
 
 /// Called when the surface is comitted