(check-sat-using sat) Assertion violation at ../src/ast/ast.h 932

Hi,
For this formula,
```
(assert (exists ((a Real)) true))
(check-sat-using sat)
```
z3 throws out an assertion violation:
```
ASSERTION VIOLATION
File: ../src/ast/ast.h
Line: 932
is_app(n)
(C)ontinue, (A)bort, (S)top, (T)hrow exception, Invoke (G)DB
```

OS: Ubuntu 18.04
Revision: 3ec7146