Exception while adding moc build steps if QTDIR is only defined in a user macro (property sheet)

When a Qt version variable is defined only in a user macro and is used in the vcxproj file (ex: Qt5Version_x0020_x64="$(QTDIR)"), the extension throws an exception in AddMocStep (QtProject.cs) when mocing because "VersionInformation.Get(versionManager.GetInstallPath(envPro))" (line 837) returns null (since commit 828f4d86) and we try to access it later to set the variable "mocSupportsIncludes" (line 838).

This is a problem for us because the Qt version variable "$(QTDIR)" is defined in a user macro with a property sheet (multiple Qt paths for static/non-static with x86/x64 build configs) and the extension doesn't use that macro when it tries to find which Qt version is available while adding the moc build steps.