input: Write a python function to generate a bar chart from a DataFrame using matplotlib.
  assert isinstance(generate_bar_chart(pd.DataFrame({'Category': ['A', 'B', 'C'], 'Values': [10, 20, 30]})), type(None))
  assert isinstance(generate_bar_chart(pd.DataFrame({'Items': ['X', 'Y', 'Z'], 'Amount': [5, 15, 25]})), type(None))
  assert isinstance(generate_bar_chart(pd.DataFrame({'Product': ['P1', 'P2'], 'Sales': [100, 200]})), type(None))
output: import pandas as pd
  import matplotlib.pyplot as plt
  
  def generate_bar_chart(df):
      df.plot(kind='bar', x=df.columns[0], y=df.columns[1])
      plt.xlabel(df.columns[0])
      plt.ylabel(df.columns[1])
      plt.title('Bar Chart')
      plt.show()