Error message 'Assets file ...\project.assets.json' doesn't have a target for ...

**Visual Studio Version**:
16.9.3

**Summary**:
Errors when trying to build.

**Steps to Reproduce**:

1. Create a console app (ConsoleApp1) for .net 5.
2. Change to multi-target .net 4.72, .net 3.5
3. Add a conditional property group in order to rename the output assembly name for one target
```

Exe
net472;net35

ConsoleApp35

```

**Expected Behavior**:
Build successfully.
.NET 4.7.2 output should be ConsoleApp1.exe
.NET 3.5 output should be ConsoleApp35.exe

**Actual Behavior**:
Fails to build with errors

**User Impact**:
Annoying