Fix crashes when operating on empty folder (#216)

widgets/account.go

@@ -1,6 +1,7 @@
 package widgets
 
 import (
+	"errors"
 	"fmt"
 	"log"
 
@@ -170,8 +171,11 @@ func (acct *AccountView) SelectedAccount() *AccountView {
 	return acct
 }
 
-func (acct *AccountView) SelectedMessage() *models.MessageInfo {
-	return acct.msglist.Selected()
+func (acct *AccountView) SelectedMessage() (*models.MessageInfo, error) {
+	if len(acct.msglist.Store().Uids) == 0 {
+		return nil, errors.New("no message selected")
+	}
+	return acct.msglist.Selected(), nil
 }
 
 func (acct *AccountView) SelectedMessagePart() *PartInfo {

widgets/msgviewer.go

@@ -2,6 +2,7 @@ package widgets
 
 import (
 	"bufio"
+	"errors"
 	"fmt"
 	"io"
 	"os/exec"
@@ -211,8 +212,11 @@ func (mv *MessageViewer) SelectedAccount() *AccountView {
 	return mv.acct
 }
 
-func (mv *MessageViewer) SelectedMessage() *models.MessageInfo {
-	return mv.msg
+func (mv *MessageViewer) SelectedMessage() (*models.MessageInfo, error) {
+	if mv.msg == nil {
+		return nil, errors.New("no message selected")
+	}
+	return mv.msg, nil
 }
 
 func (mv *MessageViewer) ToggleHeaders() {

widgets/providesmessage.go

@@ -17,6 +17,6 @@ type ProvidesMessage interface {
 	ui.Drawable
 	Store() *lib.MessageStore
 	SelectedAccount() *AccountView
-	SelectedMessage() *models.MessageInfo
+	SelectedMessage() (*models.MessageInfo, error)
 	SelectedMessagePart() *PartInfo
 }