There are two things to consider:

1. As you guessed there is a phase inversion for the reflected light when it is reflected off of a medium that is more optically dense (slower speed of light).
2. You need to be clear on whether you are considering the reflected light or the transmitted light. Whatever film thickness produced destructive interference for transmitted light will simultaneously produce constructive interference for reflected light and vice versa.

There are two things to consider:

1. As you guessed there is a phase inversion for the reflected light when it is reflected off of a medium that is more optically dense (slower speed of light).
2. You need to be clear on whether you are considering the reflected light or the transmitted light. Whatever film thickness produced destructive interference for transmitted light will simultaneously produce constructive interference for reflected light and vice versa.

Destructive interference for reflection would occur for $t=\frac{\lambda}{4}$ when the film has an index of refraction between that of the two other media. For example an anti reflective coating on a lens would have a value less than that of the glass (and more than that of air). In that way there is a phase inversion at both interfaces for light starting in the air, or neither for light originating in the glass, either case resulting in an effective path difference for reflection of half a wavelength.