‘A Dream within a Dream’.

Take this kiss upon the brow! And, in parting from you now, Thus much let me avow — You are not wrong, who deem That my days have been a dream; Yet if hope has flown away In a night, or in a day, In a vision, or in none, Is it therefore the less gone? All that we see or seem Is but a dream within a dream Poe, Edgar Allan. "A Dream Within a Dream"
How does the poem "A Dream within a Dream" relate to the concept of existentialism?