newt: Fix crash when package is missing

When package from build was missing the b.Build() was returning
an error before AppBuilder was created. It was resulting in
dereferencing null pointer in such situation.

newt/cli/build_cmds.go

@@ -161,10 +161,12 @@ func buildRunCmd(cmd *cobra.Command, args []string, printShellCmds bool, execute
 		}
 
 		if err := b.Build(); err != nil {
-			if b.AppBuilder.GetModifiedRepos() != nil {
-				util.ErrorMessage(util.VERBOSITY_DEFAULT,
-					"Warning: Following external repos are modified or missing, which might be causing build errors:\n%v\n",
-					b.AppBuilder.GetModifiedRepos())
+			if b.AppBuilder != nil {
+				if b.AppBuilder.GetModifiedRepos() != nil {
+					util.ErrorMessage(util.VERBOSITY_DEFAULT,
+						"Warning: Following external repos are modified or missing, which might be causing build errors:\n%v\n",
+						b.AppBuilder.GetModifiedRepos())
+				}
 			}
 			NewtUsage(nil, err)
 		}