Validate admin targets before actions

admin.go

@@ -169,7 +169,9 @@ func (b *Barnard) AdminItemKeyPress(ui *uiterm.Ui, tree *uiterm.Tree, item uiter
 	if !ok || admin.action == nil {
 		return
 	}
-	admin.action()
+	if !b.withValidAdminTargets(admin.action) {
+		b.AddOutputLine("Admin: action target is no longer available")
+	}
 	b.UiAdmin.Rebuild()
 	b.Ui.Refresh()
 }
@@ -549,13 +551,37 @@ func (b *Barnard) handleAdminPrompt(text string) bool {
 	}
 	prompt := b.pendingAdminPrompt
 	b.pendingAdminPrompt = nil
-	prompt.action(strings.TrimSpace(text))
+	if !b.withValidAdminTargets(func() { prompt.action(strings.TrimSpace(text)) }) {
+		b.AddOutputLine("Admin: action target is no longer available")
+	}
 	if b.Client != nil && b.Client.Self != nil {
 		b.UpdateInputStatus(fmt.Sprintf("[%s]", b.Client.Self.Channel.Name))
 	}
 	return true
 }
 
+// withValidAdminTargets runs an action only while its selected targets are
+// still members of the current connection. Menu actions can outlive server
+// removal events while a prompt is open.
+func (b *Barnard) withValidAdminTargets(action func()) bool {
+	if b.Client == nil {
+		return false
+	}
+	valid := true
+	b.Client.Do(func() {
+		if u := b.adminTargetUser; u != nil && b.Client.Users[u.Session] != u {
+			valid = false
+		}
+		if ch := b.adminTargetChan; ch != nil && b.Client.Channels[ch.ID] != ch {
+			valid = false
+		}
+		if valid {
+			action()
+		}
+	})
+	return valid
+}
+
 func (b *Barnard) CommandAdmin(ui *uiterm.Ui, cmd string) {
 	b.executeAdminCommand(cmd)
 }